Hashtags (#) differentiate Twitter from other social networking sites. They are a simple way to catalog and connect tweets about specific topics. Perhaps most importantly, however, they help filter out unrelated tweets that might randomly contain a misleading keyword or two. Hashtags are inserted into the tweet itself, usually before or after the main message. A hashtag is a single word or phrase preceded by the pound sign (#). It looks like this: #whatchawearing.
Users can search for hashtags on Twitter and catalogue tweets based on particular hashtags. You can use multiple hashtags for a single tweet. Now, that single tweet appears in multiple conversation threads, and is viewed by more people as they track and participates inthe conversations that interest them.
For example, if someone is interested in any news on impending hurricanes, they can do a search on search.twitter.com for the keyword hurricane. They can get even more specific type in Hurricane Bill. Twitter search will provide a list of tweets that have mentioned this topic, most of them organized by a hashtag such as #weather, #hurricane or, more specifically, #hurricanebill.