How do I get started?
The first thing you'll need to do is download a client application from one of the major messaging systems like AOL Instant Messenger, MSN Messenger or Yahoo! Messenger. The Resources section contains links to these IM programs, all of which are free.

Visit the IM site of your choice from the list and follow its download and installation instructions. Regardless of which one you pick, when you install an IM client, you'll need to sign up with a unique user name and password. After you've logged on with your new user name, use the integrated directory to find an IM partner or enter a friend's user name directly. Requesting an IM sesssion with another user is as easy as highlighting their name from a list and clicking a button. For more information on IM fundamentals, read the How Stuff Works article on Instant Messaging.

But what if you want to IM your friend on MSN Messenger and you only have AOL Instant Messenger? Sadly, it's not possible without additional software. But if you download a multi-system messaging program like Trillian or Gaim (see the Resources section for links) you can chat with your friends, regardless of the software they use.

I've started to IM my friends, but I can't understand half of what they are saying; what are these crazy abbreviations?
Much like e-mail, which can be pretty informal, IM takes a relaxed and streamlined approach to communication. Since the emphasis is on speed and brevity rather than adherence to formal rules of grammar, a sort of textual shorthand has emerged. Here are a few commonly used abbreviations and acronyms, but you can find many more via links in the resources section:

In addition to acronyms and abbreviations, IM users rely on symbols called emoticons to express their emotions. The most common is the smiley face :) or :-), but others include <3 (love) and :O (surprise). For more help in translating these acronyms and symbols, visit the sites listed in the resources section. Of course, there's nothing to say that you must use IM slang and emoticons when chatting with your mom in Des Moines; don't let a language barrier stop you from trying

Are there downsides to this technology?
Aside from the lack of compatability between the major messaging systems, users won't find many negatives to this technology. However, employers who choose IM as a business tool may find that messaging unrelated to work becomes a problem.

Final Thoughts
IM is fast becoming de rigueur in certain segments of our society. It's not hard to see why, as it allows people stay connected across the globe without pesky long-distance phone bills. Interestingly, some recent versions of IM software have begun to offer audio and video chat. These features, which work best over broadband Internet connections, offer even more reason to give IM a chance.

Resources

IM Services
AOL Instant Messenger: http://www.aim.com/
MSN Messenger: http://messenger.msn.com/
Yahoo! Messenger: http://messenger.yahoo.com/

IM Programs which will work with multiple services
Trillian Basic: http://www.ceruleanstudios.com/learn/
Gaim: http://gaim.sourceforge.net/

Wikipedia: Comparison of Instant Messengers
http://en.wikipedia.org/wiki/Comparison_of_instant_messengers

IM language guides
Sharpened Glossary of E-mail and Chat Acronyms: http://www.sharpened.net/glossary/acronyms.php
Bronwen & Claire's Really Huge Emoticon Collection: http://www.angelfire.com/hi/hahakiam/emoticon.html

How Stuff Works: Instant Messaging http://www.howstuffworks.com/instant-messaging.htm