A topic with a hash symbol (“#”) at the beginning of a word or phrase identifies what is after the hashtag. Twitter hashtags like #followfriday help spread information on Twitter while also helping to organize it.
The hashtag is good for organizing events or a popular trending topic. If all users begin to use hashtags it will organize Twitter.com a whole lot better. It becomes easier to find that topic in search and is more likely the topic will appear in the 'Twitter's Trending Topics'.