I'm hesitant to open this can of worms, but you are correct in thinking that 4chan is simply and imageboard. The DDoS attacks on MasterCard were actually launched by a group called Anonymous, which is often associated with 4chan. However, 4chan and Anonymous are completely separate entities and their names should not be used interchangeably.
Users on 4chan are by default anonymous when posting content or comments (replies to content). At least when I last spent time there (in early 2007) it was considered a faux pas to post content with a username as it is looked as if one is trying to gain notoriety/fame for themselves, instead of giving up their content for the great good that is Anonymous.
Anonymous (capitalized) is, generally speaking, not a coherent group in any sense, but a meme describing the swarm of memes and the users who propagate them (often first at 4chan) through the internet (a meta-meme?).
I think 4chan can be thought of as the gathering place for people who organize movements like Anonymous (and for other folks who want to look at porn and weird photos). Saying "4chan hackers" launched an attack is like saying "HN readers" drive entrepreneurship in the US, it has a bit of a truth but (1) the readers of 4chan is a dynamic bunch, there is no clear set of "hackers" associated with it and (2) not all hackers within Anonymous are associated with 4chan.
I've only ever visited the site once or twice, but that's pretty much all it is. I'm pretty sure it's just a message board (a really painful one to look at). I'm guessing there are small groups of people that collaborate using it, though. These groups of people with nothing better to do usually like finding obscure sites or community networks to "represent".
