I have 3 twitter accounts - each has a different area of interest.... the newest one I just created for poker.
Twitter is like an RSS feed only better. Twitter is more like the shoutbox... and you can Twitter via cellphone, browser or desktop widget.
I use Twitter via a desktop widget (twhirl). Here's a screenshot

1, 2, 3 are posts... like text messaging on you cellphone and 4 is the box you enter your message. Post 1 was made by someone who is in my Twitter group (I am following) so I see their posts. Posts 2 and 3 were made by me and anyone that is in my group (following me) sees the post.
Why would you want to be on twitter? I use it everyday to keep in touch with friends, family and new people that have the same interests that I do. What are you interested in? What ever it is people are talking about (search on twitter and find things you are interested in) its on twitter. Hobbies, celebrity gossip, news, business... list goes on and on
Poker - I post reminders of the nights games, links to new posts and little poker tidbits. I'm usually on Twitter all day - got a question - easy to ask.
News - twitter has the news faster than its available anyplace else. When the plane crashed - someone immediately twittered that he was on a ferry and the plane went down right in front of them... he than kept everyone updated as to what was happening, as it happened.
Facebook and Myspace require too much time and effort.Twitter is a mini-blog (the message length maxiumum is 140 characters) I prefer twitter to IM, Facebook and Myspace. Twitter just sits in the corner of my monitor like a news ticker... and I can jump in at anytime or I can just be a passive observer.
Give it a try ... any questions... just ask.