Just a way to get your tweet noticed by assigning it a theme. Remember, everything about Twitter is "LOOK AT ME! LOOK AT ME!" so anything that gets more people to read your mundane daily crap is used by all who tweet.
Ignore the comments from Twitter haters that don't even use the service.
The # is called a hashtag and it's used to help group related information so it can be found with twitter search. You often see hashtags for specific topics displayed on the bottoms of TV shows these days to help twitter members join discussions about what they're watching and the topics being discussed.
Think of it like keyword tags in blogs, same concept really.
The # symbol, called a hashtag in Twitter, is used to mark keywords or topics in a Tweet. It was created organically by Twitter users as a way to categorize messages. A hashtag is a way to bring together Tweets on the same subject. It is simply a way for people to search for tweets that have a common topic.
1) People use the hashtag symbol # before relevant keywords in their Tweet to categorize those Tweets to show more easily in Twitter Search.
2) Clicking on a hashtagged word in any message shows you all other Tweets in that category.
3) Hashtags can occur anywhere in the Tweet.
4) Conversational or commentary
Have a great day ahead!
[edited by: engine at 4:34 pm (utc) on Jul 2, 2012] [edit reason] no promo sigs, thanks, see WebmasterWorld TOS [/edit]