Using this symbol (without a space) allows you to place your tweet into a Twitter stream that is filled with other tweets sharing that same topic.
For example, #Facebook in your tweet will place it amidst countless other tweets that are discussing that social networking site in some way.
Why should you care?
- Hashtags help driving trending topics -- when breaking news is happening, the hashtag symbol allows all sorts of folk who don't follow each other to share information.
- Hashtags will help you find tweets that you find interesting on Twitter.
- Hashtags will help strangers (and new followers) find you.
- Hashtags will help your tweets appear in Twitter Search.