What does # mean on twitter?

[s0me0ne]

I know that @(username) means reply but what does #(something) mean on twitter?

[David D]

It allows the word to be searched for without finding every instance of the word, just those marked with a hash.

[mareeyum]

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.

[neofactor]

It means a HASH tag... which is like a keyword you are referencing...
Check this out... Twitter hash tags: http://hashtags.org/

They usually are short names for things... it is a way to tag your post to be associated in a group... when you search for them.

I use it all the time when talking about something like I am headed for a #run or I am at the SouthBySouthWest event #SXSW

[Chris =)]

usually means number