Some of them are. The Guy Fawkes masks are sort of a good way of describing them: a bunch of completely unrelated people assuming the same identity for a time. Likewise, the "Anonymous" you hear of is usually the "Anonymous" that pulls this sort of stunt and then publicizes it. There are a number of people hanging out on /b/ doing nothing but humorous (depending on your sense of humor) image manipulation, also calling themselves Anonymous, and people trolling LiveJournal doing the same. They've all got a different character, but if they all use the same name, it makes them difficult to attach attributes to.

They get to be anonymous by all assuming the same name, "Anonymous"; it's tricky to talk about them as a unified group because it's a group of groups, all with the same name. "This Anonymous" versus "that Anonymous" is hard to talk about. (It's a disclosed exploit in language.)