Many of the same conventions that apply to newsgroups and e-mail are also relevant in chatrooms. You're still just typing your thoughts and sending them to people, but in a chatroom, they're just a little quicker to respond.
Since there are a wide range of chatrooms available on the Net, it also follows that there are different standards of behaviour depending on your particular group. Something that may be perfectly OK in one chatroom, like using profanity, may be totally unacceptable in another.
There are some universal conventions that will be good advice no matter where you're chatting.
Avoid Using ALL CAPS
Still considered yelling. You'll be quickly scolded. Increasing your font size or typing in bold is just as bad.
Decide What Tone the Conversation Has Before Posting
Don't use offensive language or nicknames, unless you're in a chatroom that likes that kind of thing. Always avoid making personal attacks and calling names.
Don't "Flood" the Chatroom
Flooding is repeating messages over and over, or filling the screen with gibberish, in order to impede communication in the chatroom. This kind of behaviour belongs back in grade school, and that's whom you'll be associated with if you like to flood chatrooms.
Don't Flirt with Everyone in the Chatroom
This should go without saying, but we've all had to endure far too many annoying chatroom come-on's. Save the promiscuity for one-on-one correspondences please.
Be Nice to Newbies
We were all there once. If someone stumbles into your chat, who is obviously new to this medium, show them some patience and help them get the hang of it.