[Haskell-cafe] Memory and Threads - MVars or TVars

Hi everyone. I was wondering if someone could just guide me toward some good information, but if anyone wants to help with a personal explanation I welcome it. I'm trying to write a threaded program and I'm not sure how to manage my memory. I read up on MVars and they make a lot of sense. My real question is what is "atomic" and how does it apply to TVars? I don't understand what atomic transactions are and I can't seem to find a concise explanation. I also saw some stuff about TMVars? But I can't find much on them either. Any help would be appreciated. -Eitan