IM is instant messaging. There are many different IM protocols, such as MSN, Yahoo!, AOL and ICQ. One popular IM client is gaim.
Gaim Download (choose the .exe file if you're on windows)
If you want to make sure people in the middle aren't reading and logging your IMs, which they usually are, check out OTR.
If you're looking more for group chat, or to chat with strangers, chekc out IRC.