There are two simple abbreviations to know:
BBY - Before the Battle of Yavin. The Battle of Yavin is the classic battle in A New Hope (Ep. IV), during which the Death Star is destroyed. Thus, 10 years BBY takes place ten years before the original Star Wars movie (Ep. IV).
ABY - After the Battle of Yavin. Thus, Return of the Jedi (Ep. VI) takes place 4 ABY, or 4 years after A New Hope (Ep. IV).
For my timeline reference, up to date book placement, and help with the Era descriptions, I primarily use http://www.timelineuniverse.com/StarWarsTimeline.htm