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.
Hashtags is a great way to follow community driven news chatter on Twitter. Some tags are temporary in nature, while some tags are here to stay because they have been adopted by grass roots movements. This Wikia is meant to help you navigate all the hashtags on http://hashtags.org/, to figure out who is using them.