Rogue One is a stand-alone film set in the Star Wars universe. Unlike the 7 Episodes we have seen, Rogue One simply fills the gap in knowledge of what happened between Episode III and Episode IV, rather than further the plot of the Episodes so far.
Essentially, the basis for Rogue One is:
the tale of how the Rebellion acquires the Death Star plans in Episode IV and discusses why there was such a major flaw in the design of the Death Star
It is part of the Star Wars Anthology Series which was planned after the decision to make the Sequel Trilogy.
The next anthology film, based on how Han Solo
becomes the criminal smuggler we meet in the Mos Eisley Cantina
is set to come out in May 2018, and will be directed by Phil Lord and Christopher Miller
The final anthology film, which, not much is known about, has had a writer for the script since September 2016 and is planned to be released in 2020
The difference between these films and the Episodes is that they don't further the plot of the Skywalker family and the story of the Jedi, however they give us more information about plot points and characters we have only been able to speculate about and expand the galaxy.