They aren't a group as such. Anyone can claim the banner as a member of Anonymous, post videos, etc. as if they were the head of the group. But there is no head. Some of the stuff some members that choose to consider themselves members of Anonymous do some beneficial social activism, but there are quite a number of others that are willing to pull a Lulzsec or simply hack groups to increase in popularity/get respect. The problem with all this is that they don't have any clearly defined goals, and anyone can try and make their ideas those of the group to legitimize it. Not always a good thing.
For a good example of this, check out the Anonops twitter page. They're a smaller, more directed unit of Anonymous, and they've tweeted "TO PRESS: MEDIAS OF THE WORLD... STOP LYING! #OpFacebook is just ANOTHER FAKE! WE DONT "KILL" THE MESSENGER. THAT'S NOT OUR STYLE #Anonymous". It's some random guy or group claiming to speak for all of Anonymous.