While details are at this time scarce, it would seem that the fundamental story of Rogue One: A Star Wars Story will be about a group of rebels stealing the plans for the Death Star.
At the end of Episode II we see the plans in the hands of Count Dooku being delivered to the Emperor, at the end of Episode III we catch a glimpse of it being constructed and by Episode IV the Death Star has been completed.
Episode IV: A New Hope is essentially how Princess Leia passes the plans onto R2D2 for safe-keeping and this droid is then taken to the rebels where the plans are used to bring down the Death Star. How these plans ended up in Leia's hands is basically the premise of Rogue One.
It was revealed that Rogue One will take place between Episodes III and IV, being closer to IV in the timeline. As such it is an indirect prequel to A New Hope.
Style-wise it would seem that it will differ a bit from the original Star Wars story-line movies. Director Gareth Edwards (Godzilla, Monsters) has a darker tone to his movies, while still being very character-driven. Early notes reveal that it will bring the 'war' into 'Star Wars' but also be somewhat of a heist movie. As such it is also without Jedi and any of the main Star Wars characters, although there are unconfirmed rumors of Darth Vader appearing, or at least Grand Moff Tarkin.
The departure from the main movies is a welcome element for many fans. It is the first time that the Star Wars universe is being explored in motion picture format (besides the TV and print expansions), no doubt showing elements that have thus far been in the storytelling shadows.
It is also the first in a series of anthology films that will take place within the Star Wars universe at different points in time. Other anthology films in the pipelines are a young-Han Solo film, a possible Boba Fett film (and/or a Han Solo/Boba Fett movie) and a suggested Obiwan Kenobi film series, among others.