It really depends on the class itself. The most common thing is that the "ending" is determined by a single choice near the end of your class story. Keep in mind when I say "ending" I mean variants on the final class events that will likely be referenced in any future class expansions. For the most part the different endings are the same with some minor differences.
Below is what I can recall for specific classes(put in a spoiler block to be safe but I have tried to keep any spoilers to a minimum):