What does instant messaging software allow you to do? Basically, it allows two people to communicate in "real-time" over the Internet. Email, while it can be a lot quicker than postal mail or FedEx, sometimes isn't quick enough. You may need to talk to someone "right now." Instant messaging software allows you to talk to keep track of your friends by letting you track when they log on and off. You can send short messages to each other "instantly." Some of the software packages also let you send files back and forth as well.
Instant Messaging is not really a new concept. Back in the old BBS days, you could have real-time chats with the System Operator (basically, the guy who ran the BBS). Things like the Unix 'talk' program and IRC have been around for a long time. 'Talk' is basically a two-person text chat session. In order to use it, you need to know what computer a person is logged into and what 'user' they are logged in as. This is more difficult as people are using their own PCs and most providers use "floating" IP addresses. IRC works better, but it's not all that intuitive to use and the various IRC server networks have their share of problems.
Instant messaging basically works on somewhat of a client/server model. When you run the client, it connects to a central "server" that verifies your identity and registers you as being online. Other users, once they register and connect, will know that you are logged in because the server will know that you are online. Depending on how the instant messaging software is implemented, communications can take place either thru the server or directly from client to client.
One of the problems with the instant messaging software that's out there now is that the programs use incompatible protocols and do not interoperate with one another. For instance, an ICQ user can not use his software to talk to someone using AOL's Instant Messenger. Unless you're like me and signed up with the various companies that offer instant messaging, there's no guarantee that you'll be able to talk to anyone because they may have chosen one or the other. Standards need to be established so all these software packages can interact with one another and instant messaging can become as ubiquitous as email.
There's also the flip side of using this software. Now I'm "too easy" to contact when I'm online if these programs are running on my computer. Most of the time, I don't have these programs on unless I specifically want to talk to someone as they can all run in the background and let you know when someone desirable logs on.
Here are three different Instant Messaging software packages I've played with.
ExcitePAL
http://pal.excite.com/go.webx?98@@pal/index.html
AOL Instant Messenger
http://www.aol.com/aim/