What's that # thing everyone is putting at the end of their statuses on Facebook?

[djslfk;jaois]

For example:
40 cents off per gallon at Safeway today and filled up for $33! #doublewinning

[Kayleb]

not all the way sure but i think its a charlie sheen quote

[Dani]

It's called a hash tag. It started on twitter I think? Like you could be all #bored and everyone that put that as their hashtag will pop up and you can find people with similar interests or whatever. Now apparently businesses are using them ^^^^^^