IM

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)

Once you have gaim installed and setup so you can login comfortably:

download OTR which is an encryption plugin. You'll want to scroll down to "OTR plugin for gaim" and get the plugin for your version of gaim. (You can see your version when in gaim by going to "help: about")

If you're looking more for group chat, or to chat with strangers, chekc out IRC.