The # symbol, known as a hashtag, is used as a metadata tag in social media services such as Facebook, Flickr, Tumblr and Twitter. This helps group large amounts of information about the same thing under one tweet or posting. This is not, of course, the symbol's original purpose.
For decades before the advent of the internet, # was known as the 'number sign,' as it often replaced the word 'number,' as in 'my favorite football team is the #1 team in the world.'
A Symbol of The Twenty-First Century
The predominate modern use for # is on Twitter, where it is a way for people to search for tweets that have a common topic; that topic might be a celebrity, a film, a news or sports event, a musical festival, a product, pretty much anything you can think of that people might want to find out more about.
Hashtags allow you to create communities of people interested in the same topic by making it easier for them to find and share info related to it.
If you posted a tweet about Blurtit, and wanted to give people the chance to find out more, you could end your tweet with '#blurtit' for example. Clicking on the phrase would allow people to access the latest tweets about Blurtit.
Here's an online tutorial explaining # further: