Definition: 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.
Example: In the Tweet below, @twitter added the hashtag before the phrase "thankyousteve". The word is now a link to search results for all Tweets containing "#thankyousteve" in the message.
The third party site hashtags.org offers an overview of popular hashtags used on Twitter. Find out about trends, look at small, pretty graphs, and search to see if the hashtags of your fantasies exist.
Check out this troubleshooting article for solutions to common problems with hashtags.