The galaxy far, far away.
The Original Trilogy: A New Hope (1977), The Empire Strikes Back (1980) and Return of the Jedi (1983)
The Prequel Trilogy: The Phantom Menace (1999), The Attack of the Clones (2002) and Revenge of the Sith (1994)
The expanded universe - everything that happend in GFFA outside the movies.
In GFFA the Battle of Yavin (A New Hope) functions as "year zero." Hence, ABY means - After the Battle of Yavin while BBY means Before the Battle of Yavin.