ICQ - I seek you. Refers to the messaging and chatter-tracking services provided by such sites as icq.com.
IM - instant messaging; instant messenger. IM is a private chat window which allows the chatter to see when the other person is online and to activate the window for chat. Usage example: "I'm online in the mornings. We could chat. Have you got IM?" IM programs include, but are not limited to, Yahoo Messenger, AIM, AOL, Trillian.
IMHO - in my humble opinion.
IP - Internet Provider.
IRC - Internet Relay Chat.
ISP - Internet Service Provider.