At first I thought it was a form of instant messaging. And while it has some of its characteristics, it's different. I think it can best be described as 'broadcasting your current status'. To people who want to see it. Like the status field you have in applications like MSN and Skype. Elizabeth described it to me as 'more like IRC', and she has a point. It's like IRC, without channels. The '/me' command on IRC is very twitterish even.

Messages are never more than 140 characters, which helps keeping them to the point.

People don't just post their status: it's used a lot for 'look at this' type messages, usually some url that someone has just read and wants to share.

What I like about twitter so far:

• It's simple; it broadcasts a message to people who want to receive it, and that's about it.

• It's open; it has API's which have led to a lot of tools and devices supporting twitter. I have Twitterrific to read messages, and can easily send them through Quicksilver. My tweets are displayed on the right of my blog using a simple wordpress plugin.
• It's addicitive. It gives you a feeling of connectedness, more even than instant messengers.

What I don't like about twitter so far:

• Twittering via SMS is too intrusive; twitter with my mobile phone's browser is decent, but requires me to login each time (but maybe that's a problem with my phone, not twitter).

• There's the danger of information overload. Some people twitter so much that they easily dominate the twitter screen (example: going to the zoo, coming back from the zoo, looking at the pictures from the zoo, discovering there are a lot of pictures from the zoo, filtering the pictures from the zoo, uploading the pictures from the zoo, announcing the pictures of the zoo on flickr and all the tea-drinking that happened in between; hi Skoop! ). I can see this becoming overwhelming as I follow more people.
• Direct messages are inconvenient, I can't see my outgoing stuff; but then again, that's not what it's for.
• I miss a way to channel messages; some things are only relevant to my friends from the PHP community, some only for my coworkers, etc. But with twitter you only have individual messages and messages to all.

In any case, it's interesting so far. Let's see how I feel about it in a week or two.