Twitter is a social updating site that is based on status. Once you've created an account you can upload short 140 character updates about what you are doing and where you are at. You can also "follow" (subscribe to) other users and keep up with what they are doing.
This is best used in conjunction with your cell phone, where you can text in your update and all your "followers" are updated on what you just posted.
It's a cool, easy, and FREE way to keep up with each other.
You can check out my twitter page here.
Oh, and if you see a post saying that it is from TwitterBerry it is from my BlackBerry cell phone. Check out the link, and download the app if you have a Blackberry that is supported. It allows you to update without even sending a text message (it uses your data connection, though).