On Twitter, what do the tweets mean when using a "#" symbol?

[deadlyserious]

I'm sorry if it doesnt make sense, but I see a lot of "tweets" with like

#moonfruit or something and its weird?

I don't get it.

[katieiswow]

The hash tag (#), is used on Twitter for users to categorize their tweets. By using hash tags that place your tweet into an appropriate category, it would be easier to find by users who know how to search for hash tags.

[jamie]

It means you're too thick to use the internet.