I recently got an email from someone with a question about the difference between a hashtag (#) and a reply (@). See my reply below in bold, and you can also follow Mike Volpe on Twitter.
Sent: Monday, October 20, 2008 11:57 AM
To: Mike Volpe
Subject: RE: Twitter
Sorry to bother you but you seem to be a "goto" guy on twitter matters. I noticed in your tweet last week regarding Seth Godin's book, you had a reference to #hubspot as opposed to @hubspot. What is the significance of the hash mark? Where did I miss that information?
What's the difference between #mvolpe and @mvolpe?
Check out http://www.hashtags.org/. Basically, #hubspot indicates a threaded discussion on a TOPIC, while @hubspot is a directed message toward a specific person/entity. Think of "#" as like "about" or "re:" and @ as like "to". Typically people will search on twitter for #hubspot to follow a conversation, but not as much for @hubspot.