Sort of. The general continuity, or "canon" of the Star Wars franchise has been streamlined to include current and future feature films, The Clone Wars, this series, and most transmedia material released after 2014. Consequently, this series overwrites much previous material that took place between Episodes 3 and 4, such as The Force Unleashed and the Dark Times comic series.
Which film characters will be appearing?
A cameo has been confirmed for Lando Calrissian (voice: Billy Dee Williams).Some confirmed cameos have already aired: Obi-Wan Kenobi (voice: James Arnold Taylor), C-3P0 (voice: Anthony Daniels) and Darth Vader (voice: James Earl Jones, for a new scene inserted into a re-aired episode).Boba Fett is unconfirmed, but likely to show up at some point.
When does the story take place?
Rebels takes place starting roughly 14 years after Revenge of the Sith, and roughtly five years prior to Star Wars (A New Hope).