incrediBILL - 1:16 pm on May 18, 2012 (gmt 0)
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.