how to use the # on Twitter?

[Anna]

i see a lot of people posting things with #. i dont get what the ydo or how to use them. examples of ones i have seen : #OnMyMind #someday #NHLawards #jackischill #moam #longday

[Michael Walters]

I'm fairly new to Twitter myself, and I had the same issue. They are trends. If enough people post the same message after the '#' they become trending topics (popular topics that a lot of people are tweeting about).

[Crack]

It works like this. For example, my Twitter username is despotovski01, and to access my profile outside from Twitter, you should go to this URL: http://twitter.com/#despotovski01. This is how it works. Just change #despotovski01 with the other guys' URLs.

[Ben]

is used to signal the topic of he post. It makes it easy for people interested in opinions on something to notice it, and it means that they won't get any false matches (people who put the word in their tweet but aren't actually talking about that)

[Dest]

Pretty much what a couple of other people said.
I live in Adelaide, so when I want to see what other people are writing, I'll search for #adelaide. When I want to share important info, I use the #adelaide hashtag.

[Gajdio Asyhfkljkm]

basically, these are trends. if you look on the right hand it side, it tells you the most popular trends in your area.
for example one popular one at the moment is "#itssoannoyingwhen" so. in yoour tweet you could put
"#itssoannoyingwhen my mom makes me clean my room" then, you could press the "#itssoannoyingwhen" because it would be a link, and see what everyone else is putting.
so someone elses might be "#itssoannoyingwhen it rains as you're just about to go out" and basically yeah, its like mentioning someone in a tweet but it isnt a person its a thing. You're not allowed to put spaces in because theyre not gonna work if you do. hope this makes sense hehehee./

more examples...
a celeb might have a saying like n-dubz have "nananiii"
so in there tweets they might put "i love all my fans #nananii"
and their followers might put in there tweet "i love n-dubz #nananiii"
and if n0dubz were to search the whole of twitter for people who put "#nananii" in there tweets and perhaps RT(retweet) them.

[Marwan]

The # symbol, called a hashtag, is used to mark keywords or topics in a Tweet. It was created organically by Twitter users as a way to categorize messages.

People use the hashtag symbol # before relevant keywords in their Tweet to categorize those Tweets to show more easily in Twitter Search.

Clicking on a hashtagged word in any message shows you all other Tweets in that category.

Hashtags can occur anywhere in the Tweet.

Hashtagged words that become very popular are often Trending Topics.