I imagine many don't know why, so I'll explain it here.
All the current major chat networks are owned by companies. AOL, Yahoo, or Microsoft. These companies have free reign to do whatever they want with the network. They could censor people. They could log chats. They could boot people off. Part of their success is that they haven't been doing anything obviously nasty - but some of us, myself included, don't really like the idea that they could.
E-mail, however, doesn't have this problem. There is no central Email-Corp. AOL could obviously intercept any emails sent to or from aol.com, but if I'm sending an email to, say, Yahoo, they can't do a thing about it. Email servers are distributed, peer-to-peer, and relatively easy to set up. Anyone with a little technical knowledge can set up their own server.
You may be saying "well, so what, I don't have that technical knowledge, why should I care" - but the important part is that people who *do* have that technical knowledge can provide services to you that the big corporations might not be interested in. I, for example, own pavlovian.net, and get all my email through that - mailsnare.net gets paid a small yearly fee by me to act as my email server, but if I want to I can pull them out of the loop and put someone else in at a moment's notice.
This is pretty neat. I could set up any kind of automated service I want with this email account. I have 100% full control over it. I don't have 100% full control over "my area" of AIM - I can't fence off an area of AIM accounts and say "these are mine, do not touch", I can't provide paid AIM services because AOL might object.
Now, on to Jabber. Jabber is also a decentralized server. Anyone can set up a Jabber server, and it can talk to any other Jabber server. I, for example, have a Jabber server up at pavlovian.net (although it seems to be a kind of glitchy one, but we'll ignore that for now.) I can talk to anyone else on any working Jabber server.
Like, say, Livejournal's.
Or Google's. (Google Talk, for example, is a Jabber server.)
So now, anyone who has Google Talk or Livejournal can download another chat client and talk to me, completely bypassing AOL, MSN, and Yahoo. And if they want, they can set up their own Jabber server and bypass every large company on the planet.
Just our two servers . . . and we can add any feature we want to that. Secure chat, so it can't be snooped on. Video conferencing that works any way we want. Conference calls. Hell, we could set up a P2P network if we wanted to implement it (good luck doing that through AIM.)
And that's neat.
Power and privacy.
Once this is stable, go find a Jabber client and use that to talk to your friends. The more people do this, the faster Jabber can become the new standard.