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For all types of derivative works, namely:

• Movies


• Novels


• Comic books


• 
  Other written works (including RPG materials)


• Cartoons


• Video games


• 
  Derivative movies (such as the infamous Ewok movies and even more infamous Holiday Special)

According to what rules does something become "canon" and who is responsible for the evaluation of a published work?

Canonicity in the Star Wars universe is, as of April 2014 determined by a working group comprised of representatives of Disney and LucasFilm known as the Lucasfilm Story Group.

The primary change made is that the old canon system (G-Canon, T-Canon, etc) has been nuked from orbit and only the original six feature films (the Original Trilogy and the Prequel trilogy, as seen in the 2011 Blu-ray Edition), Star Wars: The Clone Wars TV show, Star Wars: The Clone Wars film and Star Wars: Rebels TV shows are considered to remain part of the official Star Wars canon.

All other properties, with a few small exceptions are now lumped together under a single banner known as "Star Wars : Legends". Those exceptions seem to include the film novelisations (where they specifically elaborate on things that are seen on screen) and the Star Wars Databank (which replaces the old Databank/Encyclopedia website).

"While Lucasfilm always strived to keep the stories created for the EU
consistent with our film and television content as well as internally
consistent, Lucas always made it clear that he was not beholden to the
EU. He set the films he created as the canon. This includes the six
Star Wars episodes, and the many hours of content he developed and
produced in Star Wars: The Clone Wars. These stories are the immovable
objects of Star Wars history, the characters and events to which all
other tales must align." _{^{Starwars.Com - THE LEGENDARY STAR WARS EXPANDED UNIVERSE TURNS A NEW PAGE}}

By definition, the other properties that were originally deemed to be "G-Canon" and "T-Canon" (the Star Wars Radio Dramatisations, Star Wars Holiday Special, earlier Star Wars: Clone Wars TV Show, Ewoks films, Star Wars : DroidsTV show and Star Wars: Ewoks TV show), along with any deleted scenes (from the theatrical films), earlier theatrical versions of the films, commentaries, interview quotes, making-of documentaries, behind-the-scenes documentaries, concept art books and previous versions of the scripts are now all considered to be "Legends" properties. Fan-made properties and parodical works (such as the Star Wars Lego series and Family Guy specials) remain completely outside the canon framework

Moving forward, all future properties (films, books, comics, games and short stories) will be licensed and fully compliant with the Star Wars Story Group's rules regarding canon status, unless explicitly stated. This obviously excludes future prints of old titles which will be clearly tagged with the "Legends" banner.

Excluding the exceptions listed above, the full list of canon works (past, present and near future) now stands as follows.

Films

• 
  Episode I: The Phantom Menace
  (and the novelisations by Terry Brooks and Patricia Wrede)


• 
  Episode II: Attack of the Clones
  (and the novelisations by R. A. Salvatore and Patricia Wrede)


• 
  Episode III: Revenge of the Sith
  (and the novelisations by Matthew W. Stover and Patricia Wrede)


• 
  Episode IV: A New Hope
  (and the novelisations by Alan Dean Foster, Ryder Windham and Alexandra Bracken)


• 
  Episode V: The Empire Strikes Back
  (and the novelisations by Donald F. Glut, Ryder Windham and Adam Gidwitz)


• 
  Episode VI: Return of the Jedi
  (and the novelisations by James Kahn, Ryder Windham and Tom Angleberger)


• 
  Episode VII: The Force Awakens
  (and the novelisations by Alan Dean Foster, Michael Kogge and Elizabeth Schaefer)


• 
  The Clone Wars
  (and the novelisation by Karen Traviss)


• 
  Rogue One (and the novelisations by Alexander Freed and Matt Forbeck)

TV Shows

• The Clone Wars: Season 1-5


• The Clone Wars: The Lost Missions


• Star Wars Rebels: Spark of Rebellion


• 
  Star Wars Rebels: Season 1-3 (ongoing)

Novels

• 
  Lords of the Sith by Paul S. Kemp


• 
  Tarkin by James Luceno


• 
  A New Dawn by John Jackson Miller


• 
  Servants of the Empire: Edge of the Galaxy by Jason C. Fry


• 
  Ezra's Gamble by Ryder Windham


• 
  Dark Disciple by Christie Golden

Comics

• 
  Kanan: The Last Padawan by Greg Weisman


• 
  Star Wars by Jason Aaron


• 
  Star Wars: Darth Vader by Kieron Gillen


• 
  Vader Down by Jason Aaron and Kieron Gillen


• 
  Lando by Charles Soule


• 
  Poe Dameron by Charles Soule


• 
  Shattered Empire by Greg Rucka


• 
  Han Solo Han Solo by Marjorie Liu


• 
  Chewbacca by Gerry Duggan


• 
  Princess Leia by Mark Waid


• 
  Star Wars Special: C-3PO by James Robinson


• 
  Darth Maul: Son of Dathomir by Jeremy Barlow


• 
  Heir to the Jedi by Kevin Hearne

Other formats

• 
  (Webcomic) Star Wars by Hong Jac Ca


• 
  (Video Game) Uprising
  


• 
  (Magazine) Star Wars: Insider short stories (after edition #150)

There are also some 40+ novels, junior novels and reference books that have been released under the banner of Star Wars: Journey to the Force Awakens, Star Wars: Journey to the Last Jedi as as well as a large (and increasing) number of books that are being released to coincide with the launch of Star Wars 7, 8 and 9 and stand-alone Star Wars films (Rogue One and Solo). This includes a number of factbooks such as 'Absolutely Everything You Need to Know', 'Ultimate Star Wars', 'The Force Awakens: Incredible Cross-Sections', 'The Force Awakens: Visual Dictionary', 'Ships of the Galaxy' and 'Star Wars in 100 Scenes' along with novels such as 'Aftermath' and 'Lost Stars' and 'Before the Awakening', etc.

Broadly speaking, any book or property published after April 2014 is considered to be canon unless explicitly stated not to be.

There's a Wikipedia article on this: Star Wars canon.

It says:

The Star Wars canon consists of the six Star Wars feature films, along with all officially licensed, non-contradicting spin-off works to the six films. As once defined by Lucas Licensing:

We have what we call Canon, which is the screenplays, novelizations, and other works that are directly tied into continuity, and then there are a lot of marginal things, like the old Marvel Comics series, that we don't really try to work into the continuity when we're planning new projects. Even the LucasArts interactive games have a premise, a backstory with player characters that we're trying to tie into the overall continuity. It is sort of a godlike undertaking. We are creating this universe as we go along, but somebody has to keep his finger on everything that came before.

— Allan Kausch, from The Secrets of Star Wars: Shadows of the Empire

Later in the article there's a further breakdown, between what Lucas thinks of as canon and the Expanded Universe, and another level defined as G-canon, T-canon, C-canon, S-canon, and N-canon.

I went looking and found a number of things that might answer your question -

Canonicity

When there are multiple "official" works or original media, the question of what is and what is not canonical can be unclear. This is resolved either by explicitly excluding certain media from the status of canon (as in the case of Star Wars), by assigning different levels of canonicity to different media (as was in the case of Star Wars before the franchise was purchased by Disney), by considering different but licensed media treatments official within their own continuities but not across them or not resolved at all. The use of canon is of particular importance with regard to reboots or re-imaginings of established franchises, such as the 2009 Star Trek film, because of the ways in which it influences the viewer experience.

The Star Wars canon originally existed on several levels. The highest level was the original Star Wars films, and statements by George Lucas; tie-in fiction from the Star Wars expanded universe had a different level of canonicity. The complex system was maintained by Leland Chee, a Lucasfilm employee.

After Disney bought the franchise, all material published prior to April 25, 2014 that was not any of the Star Wars movies or the CGI cartoon The Clone Wars was declared "Legends" continuity, marking them as no longer official canon. All subsequent material exists on the same level of canon, with the Lucasfilm Story Group being established to ensure no contradictions among canon works.

Disney acquisition, canon revision and Star Wars Legends

On October 30, 2012, Lucasfilm was sold to The Walt Disney Company for $4.05 billion. After the acquisition, Disney and Lucasfilm established the Lucasfilm Story Group, a committee whose job is to keep track of and define the "canon" in an effort to unify the films, comics, and other media with the existing canon.

On April 25, 2014, Lucasfilm and Disney removed all Expanded Universe works from the canon. They announced that the existing seven films and The Clone Wars television series are the "immovable objects" of Star Wars storytelling. Previously published material has been relabeled under a "Legends" label, and future content will present a different vision of people, places and events after Return of the Jedi. They also announced that all future Star Wars stories will be connected and on equal canon level as the films, with guidance coming from the Star Wars story group. Additionally, it was announced that the films of the sequel trilogy will not follow the same story as that of the Expanded Universe works taking place after the events of Return of the Jedi.
The first official canon novel in the revised continuity was Star Wars: A New Dawn by John Jackson Miller, which is a prequel novel to Star Wars Rebels. It was released September 2, 2014.

Determining canonicity (Original canon)

The Star Wars canon was first defined in a 1994 interview with Lucas Licensing's Sue Rostoni and Allan Kausch in issue #23 of the Star Wars Insider:.
Gospel, or canon as we refer to it, includes the screenplays, the films, the radio dramas and the novelizations. These works spin out of George Lucas' original stories, the rest are written by other writers. However, between us, we've read everything, and much of it is taken into account in the overall continuity. The entire catalog of published works comprises a vast history—with many off-shoots, variations and tangents—like any other well-developed mythology.

This policy has been further refined and fleshed out over the years. The official Star Wars website has also detailed the role of canon, expanded universe, or "EU" sources, and how they fit into overall Star Wars continuity. In a 2001 "Ask the Jedi Council" response by Steve Sansweet (director of fan relations) and Chris Cerasi (an editor for LucasBooks at the time), it was stated that:

When it comes to absolute canon, the real story of Star Wars, you must turn to the films themselves—and only the films. Even novelizations are interpretations of the film, and while they are largely true to George Lucas' vision (he works quite closely with the novel authors), the method in which they are written does allow for some minor differences. The novelizations are written concurrently with the film's production, so variations in detail do creep in from time to time. Nonetheless, they should be regarded as very accurate depictions of the fictional Star Wars movies.

The further one branches away from the movies, the more interpretation and speculation come into play. LucasBooks works diligently to keep the continuing Star Wars expanded universe cohesive and uniform, but stylistically, there is always room for variation. Not all artists draw Luke Skywalker the same way. Not all writers define the character in the same fashion. The particular attributes of individual media also come into play. A comic book interpretation of an event will likely have less dialogue or different pacing than a novel version. A video game has to take an interactive approach that favors gameplay. So too must card and roleplaying games ascribe certain characteristics to characters and events in order to make them playable.

The analogy is that every piece of published Star Wars fiction is a window into the 'real' Star Wars universe. Some windows are a bit foggier than others. Some are decidedly abstract. But each contains a nugget of truth to them.

The Holocron

By 1996, Licensing kept an in-house bible of reference materials as the volume of publications, facts, and figures grew to such unwieldy proportions that it became difficult to know everything relevant to a particular project. They finally decided something had to be done to organize the increasingly large collection of media which chronicled the Star Wars universe. A system of canon was developed that organized the materials into what was and wasn't fit for the Star Wars story.

In 2000, Lucas Licensing appointed Leland Chee to create a continuity tracking database referred to as the "Holocron".

The Holocron was divided into five levels (in order of precedence): G-canon, T-canon, C-canon, S-canon, and N-canon. These levels are no longer in use after April 25, 2014.

G-canon was George Lucas canon: Considered absolute canon, it included Episodes I–VI (the released films at that time), and any statements by George Lucas (including unpublished production notes from him or his production department that are never seen by the public). Elements originating with Lucas in the scripts, filmed deleted scenes, film novelizations, reference books, radio plays, and other primary sources were also G-canon when not in contradiction with the released films. G-canon overrided the lower levels of canon when there was a contradiction.

T-canon was Television canon: referred to the canon level comprising only the animated film Star Wars: The Clone Wars and the two television series Star Wars: The Clone Wars and Star Wars Rebels. Its precedence over C-Level canon was confirmed by Chee.

C-canon was Continuity canon: consisting of materials from the Star Wars expanded universe including books, comics, and games bearing the label of Star Wars. Games and RPG sourcebooks were a special case; the stories and general background information were themselves fully C-canon, but the other elements such as character/item statistics and gameplay were, with few exceptions, N-canon.

S-canon was Secondary canon: covering the same medium as C-canon, it was immediately superseded by anything in higher levels of canon in any place where two elements contradicted each other. The non-contradicting elements were still a canon part of the Star Wars universe. This included certain elements of a few N-canon stories.

N-canon was Non-canon: "What-if" stories (such as stories published under the Star Wars: Infinities label), crossover appearances (such as the Star Wars character appearances in Soulcalibur IV), game statistics, and anything else directly contradicted by higher canon ends up here. N-canon was the only level that was not considered official canon by Lucasfilm. Any published material that contradicted things established in G-canon and T-canon was considered N-canon.

So to sum it up - The Offical Cannon is the Star Wars movies and their immediate novels. As of April 25 2014 all works excluding the movies and Clone Wars tv show are now categorized as Star Wars Legends. All future Star Wars stories will be considered on equal canon level with the films.

Post-2014

On April 25, 2014, Lucasfilm officially revised and solidified the canon, stating that all previously released Expanded Universe works would be re-branded under the new Star Wars Legends banner, in order to ensure a flowing timeline with the release of the Star Wars sequel trilogy. They also announced that future Star Wars stories will be considered on equal canon level with the films, with guidance coming from the Star Wars story group.