what does this sign # means in twitter?

[Gia]

I see everybody post things for example "Sometimes you just have to buy a new modem. #caseclosed." What does this mean "#", this sign and why do they use it?

[Windowphobe]

This is a hashtag. The idea is to make it easier to search Twitter for related tweets by using that particular string. One that's going right now is #outdatedphrases, which has several dozen tweets associated with it. (I contributed one of them.) If you search for #outdatedphrases, you'll get the last hundred or so.