I chose the calm blue stars pattern for my Twitter background. I chose it to mirror how I was feeling at the time. It might also be interpreted as a bit boring, but that's alright with me. I believe that I've given the bones of who I am and what I do. I have a bit of paranoia about letting people know too much about me online.
As for how I'm using Twitter, right now I am in my exploration and experimental state. I'm using it to correspond with others in the library world. As I learn more, I might create 2 personas online: 1 for work and 1 for personal use. When I looked at the Twitter directory, I was amazed at the wide range of people on Twitter. It opens up a whole new world!
What is Twitter? I see it as a mixture of things, a place to send mini text-messages, a social community, and a gadget for your blog/webpage/social network site.