Twitter is a social messaging site that allows you to post updates and keep in touch with friends. You can follow what your friends are doing at that moment(provided they update often), and they can do likewise. You can also have twitter alert you to your friends updates via mobile text messages and by IM.

Another benefit to Twitter is the ability to link your blog with the service. By doing this, friends will be notified of your most recent post as an update to Twitter. This can definitely be a valuable marketing tool to spread the word about your blog.

For Wordpress I use the Twitter Tools plugin by Alex King. This plugin allows Twitter to post updates about recent blogs and to also bring your tweets into your blog. A handy tool to have.