What does the # in front of a phrase mean?

[allpies]

I have been seeing # signs in front of tons of stuff and I don't know what it means. For example at the end of the official Hunger Games trailer, in the lower left-hand corner it said #WhatsMyDistrict. If you don't know either then don't answer, but if you do know please tell me.

[Edward]

The # is called a hashtag and means you can follow that topic and read short or send short messages about it in real-time using your computer or cell phone through a program called Twitter. You'll need to sign up to use Twitter on your cell phone or computer. Basically, Twitter is a "network made up of 140-character messages called Tweets. It's a new and easy way to discover the latest news (“what’s happening”) related to subjects you care about." See the link below for more details on twitter and tweets.