What is the # sign that Tweeters do?

[WheatleyLaboratories]

Like before or after people on Twitter update a status or "Tweet" the ## sign? What does it mean?

[Arthur]

its complicated

[Arthur]

It hashtag.

A tag embedded in a message posted on the Twitter microblogging service, consisting of a word within the message prefixed with a hash sign.

[Belovedhill756]

It's a hashtag, denoting a common search term.

[likeit1]

Bacon and hash potatoes

[Gladius]

Like people said, it's called a hashtag, but more importantly, the function of it is to mark important words or subjects in the message so it's easier to search for. For example someone might say "The #flood is finally receding here but the power is still out and the roads are barely passable. #sandy". That way anyone interested in the flood or hurricane sandy can find it easily.