Hashtags are a community-driven convention for adding additional
context and metadata to your tweets. They're like tags on Flickr,
only added inline to your post. You create a hashtag simply by
prefixing a word with a hash symbol: #hashtag.
How To Use Hashtags
Start using hashtags in your tweets, preceding key words. It can
be helpful to do a little research first, to find out if the
subject you're tweeting already has an established hashtag.
Finally, track other tweets on the subjects you're interested in
(ie: those containing the appropriate hashtags) by
browsing/searching at Hashtags.org, TwitterGroups, TweetChat, TweetGrid, Twitterfall, etc. You can set it up with RSS
feeds as well.