Ah, sorry... heh.
Probably the best way to explain is to post my own profile:
http://www.last.fm/user/flamingdog/
It's basically a music community that tracks your listening habits through a winamp plugin or from their own 'last.fm' radio player, and then collates the results in all kinds of interesting ways. You can see what you're listening to most, by artist, album, track, etc. You can view the profiles of people who are your musical 'neighbours', and chat with them to discover new music.
A group is just a way for us to club together and swap music tips and such on the site itself. Can see what each other is listening to, etc. Just thought it might be fun.
I should add, I've only been a member for like 10 days, and I've already discovered four new bands through it.