The pound sign (#) on Twitter is actually called a hashtag.
In a sense it’s a way to sort of create a group of tweets for people to follow. In the example above #twiistup is an event that this person wants other folks following this event to read. #holla is just like say “hey” for more people using #holla as a hashtag. It’s kind of like using a meta tag on a website. It’s a tag for people to find your tweet.
hashtags.org shows real-time use of Twitter hashtags.You can also follow @hashtags to have your hashtags tracked. You can also track multiple hashtags using applications such as Tweetgrid. I’ve used that to track up to six and it shows you real-time results.
In most cases hashtags are used for big events such as the Iran Election (#iranelection), death of Michael Jackson (#mj or #michaeljackson), Presidential elections (#election), etc.
For more on hashtags check out the Twitter fan wiki.