Q. What does the hash tag (#) mean on Twitter?
A. The hash tag (#), is used on Twitter for users to categorize their tweets. By using hash tags that place your tweet into an appropriate category, it would be easier to find by users who know how to search for hash tags.
Ok, let's break this down a bit... First of all, hash tags can be made up by users -- it is really an "unofficial" categorization method developed by the community. So for example, if your post is about cars, you can include the has tag "#cars". This is an obvious hash tag, as most are. However, some are less obvious. For example, at the New York Auto Show, the hash tag used was "#NYIAS" (or "#nyias" - case insensitive). This tag was most likely created by the people behind the autoshow. Now, if you wanted to know what was going on at the autoshow, you can search for "#nyias" at search.twitter.com and see all the related posts, assuming users who tweeted about it used the tag.
The hash tag system is an imperfect system and used by a very small percentage of the Twitter community, but very useful in some cases.
(BTW - yes, we will be at #TWTRCON - http://www.twtrcon.com on May 31, 2009. Come see us!)