I’ve seen it asked on Facebook before. What’s up with the # signs in people’s posts? The response: It’s a twitter thing.
In case that’s not a good enough answer for you, here’s a quick rundown on the elusive # symbol. In Twitter terms it is called a hashtag. What does it mean, when used before a keyword in a tweet, the word becomes a link to search results for all Tweets containing said #hashtag in the message.
They can use used many different ways: to add emphasis, to give context, or generally to make a post richer and more interesting.