Hashtags are commonly used within Twitter tweets when a comment relates to a specific subject that other people might want to follow or participate in the discussion. It is often used at conferences; to follow the conference generally, or particular sessions within the conference. This practice allows people outside the actual ‘physical’ location of the conference to participart in some way with the delegates and sometimes the presenters, especially if the tweets are projected real time within the session, or questions from those using Twitter are being taken by the speaker.
Perhaps the best know service to allow this to take place is called Twitterfall.
A quick rundown of Twitterfall is presented in this video: