Why do people put a "#" in front of their words?

[noetic]

I'm beginning to see people put the number sign (#) in front of their words on facebook status updates. Why do they do that? What does it mean?

EX: "I'm about to #hit up the gym."

[G BluNt]

its a twitter thing

[perry8331]

Twitter thing. They usually don't put spaces in between like you did. #itcamefromtwitter

[arou42]

"...if your post is about cars, you can include the has tag "#cars". This is an obvious hash tag, as most are. However, some are less obvious. For example, at the New York Auto Show, the hash tag used was "#NYIAS" (or "#nyias" - case insensitive). This tag was most likely created by the people behind the autoshow. Now, if you wanted to know what was going on at the autoshow, you can search for "#nyias" at search.twitter.com and see all the related posts, assuming users who tweeted about it used the tag."