Twitter is, well, great. It's a weird mix of blog, instant messenger, and message board.
Like a blog, you post (or tweet, as it's known in the world of Twitter) something and it can be out there for everyone to read. However, there is a 140 character - character, mind you, not word, and including spaces - limit on an individual post. To give you an idea of what that means, this post, right here, will have used 514 characters by the time we get to the end of this sentence. So it forces you to be brief, concise, and to the point.
Yes, I said can be out there. See, you can set your Twitter account so that only people you allow can read what you say. If you want everything you tweet to be private, you can set it up that way. Even if you allow everyone to read, you can still send direct messages to another person that only that person can read. You can also reply in a manner that allows everyone who follows you (follows?) can see you reply.
And that's how it starts to merge into an instant messenger. Because of the brevity, because of being able to respond, and because of following. Following is like having a friends' list on an instant messenger service, with a big difference. See, you can follow anyone, whether you know them or not, as long as they allow you to do so. Here are some people I follow:
Greg Grunberg (who tweets, including posting pics, from the Heroes set)
as well as some author by the name of Lisa McMann, whoever she might be, and Sasha, though she never seems to tweet. There's a few others I know personally, though I'll leave it to them to out themselves if they want outed.
In addition, almost all the big names currently working for Marvel comics are on Twitter, as well as many Marvel staffers, including the current editor-in-chief Joe Quesada. It's cool as a comics fan to watch them talk back and forth at each other.
It's those discussions that bring it into message board status. One person will tweet, and a bunch of others will respond, leading to even more tweets that either stay on-topic or roam wonderfully far afield.
If you do dive in, be sure to grab one of the available desktop applications. Twitter is browser-based, but with a desktop application you can use Twitter without being in a browser, kinda like having an instant messenger service application running. I use Twhirl, and love it. Here's a list of some others, some of which allow you to update Twitter via mobile, if you're so inclined. And if you do join, here is one columnist's list of the twelve people you should follow on Twitter.
So, come on, jump in, join the fun. Try it for a week and you'll be hooked. Hell, try it for a day and you'll be hooked. If you do, follow me at Dweeze.