What does # mean on Twitter?

[ErvE]

I've been seeing things like #michaeljackson or whatnot lately on Twitter. I have no idea what this means!
Also, might as well, What is Follow Friday?

[PhonicUK]

A # is used to represent a 'tag' which is usually a topic or a keyword. Its meant as an easy way for people to search for tweets about a given topic.

[Luis]

The # symbol is a hashtag used to mark subjects or conversations about trends or activities (also known as Twitter Memes or Twemes) so they're easy to search. Say if you write a tweet about MJ and include "#michaeljackson". Any user will be able to lookup all the tweets marked identically, when they do a search for #michaeljackson.
You can find an organized list of them here: http://twemes.com/

#Followfriday is a popular Twitter Meme where users suggests other Twitter users to follow.

It's common courtesy in Twitter that when someone follows you, you then follow them back. But at one point this becomes cumbersome to read because of all the tweets that accumulate on your page. As you become more selective about who you follow, you are bound to offend some people who you didn't follow back. For that reason I choose not to participate in the Friday meme. I barely have enough time to read the tweets of the users I currently follow, I don't need more.