Defence Against the Dark Arts is a class offered at Hogwarts that, understandably, teaches the students how to defend themselves against dark magic. It has a notorious reputation of burning through teachers like a brushfire through California.
There seem to be two types of lessons that are taught in DADA: offensive/defensive spells, and protection from dangerous magical creatures, though dangerous creatures as a primary foe was mostly emphasized in the early books. This may have been because with the threat of Voldemort and dark arts all but snuffed, nasty creatures were seen as a remaining threat.
In crossover fanfic, especially crossover fic that replaces one of the books by bringing in an outside character as a teacher, expect that character to teach DADA. Though sometimes they have taken over History of Magic if they're feeling frisky.