There's a plethora of Star Wars 'Expanded Universe' material in existence (comic books, novels, video games) that takes place in the years after the rise of Emperor Palpatine and the Galactic Empire in Episode III - Revenge of the Sith and before the events in Episode IV - A New Hope. However, now that Disney owns Lucasfilm, the plan is to start fresh and generate a multi-platform experience based around director J.J. Abrams' Star Wars: Episode VII.
It's for this reason that Disney and Lucasfilm have cancelled the popular Clone Wars animated TV series, rather than move it to a different home on the Disney XD channel. Similarly, the two companies are joining forces (no pun intended) to produce Star Wars Rebels, a new (and, presumably, 3D) animated TV show that will serve as the official canon series that fills the gap between Episode III and IV.
Here is the official synopsis for Star Wars Rebels:
The action-packed series is set between the events of Episode III and IV - an era spanning almost two decades never-before explored on-screen. Star Wars Rebels takes place in a time when the Empire is securing its grip on the galaxy and hunting down the last of the Jedi Knights as a fledgling rebellion against the Empire is taking shape.
Dave Filoni - former supervising director on Star Wars: The Clone Wars - will serve as executive producer, alongside Greg Weisman (creator of Disney's popular Gargoyles TV series from the 1990s) and Episode VII consultant Simon Kinberg. The Star Wars Rebels one-hour premiere telecast is being written by Kinberg, who is also producing and writing one of the guarded live-action Star Wars movie spinoffs.
Certain members of the Screen Rant Underground Podcast team were very much fans of the Clone Wars TV series, and praised the show for offering what so many feel was missing from George Lucas' live-action Star Wars prequel movie trilogy (specifically, well-crafted storylines and compelling character development). Star Wars Rebels could recreate that success, given the collective credentials of the people working behind the scenes.
Similarly, this new animated TV show could serve as a useful bridge that helps to link the previous live-action Star Wars movies with Episode VII, in terms of finding a tone and style that's in keeping with the franchise's traditional spirit. However, this may end up being the final nail in the coffin for the live-action Star Wars TV show that would've taken place between Episode III and IV - with 50 hours of the series having already been scripted - that Lucas had planned (for better or worse).
Star Wars Rebels will premiere on Disney XD in 2014.
Source: Walt Disney Pictures/Lucasfilm