What does the little "#" mean on Twitter?

[Mess Of A Dreamer]

I recently joined Twitter, and I have the @reply thing down, but now there are pound signs everywhere. What do they mean? Any other Twitter-related tips are also greatly appreciated.

[ig4mer]

tag indicates a subject. It is usually created by someone so he/ she can organize an conversion and make it easier to follow. Without it he/she can only see the tweets of people they follow or search by keywords (which other people maybe using different keywords, spellings etc) This is especially effective in real time situation like the mumbai attacks #mumbai or swine flu #swineflu etc. To follow search #swineflu etc.

It is almost like an agreed upon keyword.

An example, if I want to join a conversation about a Mets game

I can't believe the #Mets come back last night. Great game