Hashtags on Twitter are a way to create groupings or channels. Channels can be used to filter information that relates to an event, product, service, person, geographical location. See the earlier post by Chris Messina.
Subscribe to the RSS feed for any hashtag here.
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. (www.hashtags.org)
The track facility provided by Twitter makes some # tags redundant. Eg “There’s a heatwave in Adelaide” is just as trackable as “There’s a heatwave in #Adelaide”. The # just makes the post look ugly and provides no additional context.
This was not the case with the #mumbai tag. Adding that tag allowed the tweeter room to post other more descriptive information. The #mumbai tag provided the context and the rest provided the content. Eg. “#mumbai Snipers on the roof” meant much more than “snipers on the roof”.
See also Using Hashtags and Themes for Twitter.
How to use the #tag at An Introduction to Twitter Hashtags.
And a bit more at Tweet Your Message to a Larger Audience with Hashtags (from Twitip).
Twemes is another tag aggregation service.