Dr. Alan Cann made a very decent SlideCast to help explain Twitter (embedded below):
Twitter is fun and I enjoy it, but wouldn’t enjoy it nearly as much if I wasn’t using Twhirl as a desktop client. If you try Twitter, here’s my advice:
1. Find the friends you want to try following and follow them.
2. Download and install Twhirl.
3. Leave Twhirl running in the background while you work. If you see a tweet that interests you or makes you want to respond, do. If the tweets you see don’t engage you, they’re easy to ignore after a quick glance.