TamaCHAT! is basically a chatroom where you can talk to other TamaTalk members. You can join TamaCHAT! by posting a topic in the TamaCHAT! Requests For Access forum.
TamaChat is the amazingest, randomest, dolphinniest thing on the entire Internet, but to prevent annoying people going on it who will mess up its amazingness, you have to be approved to access it. Just post a topic saying "I want to be approved for TamaChat" or something and you will be approved. Then you will be able to participate in the awesome randomness of chat!