Twitter has elevated the humble pound sign, or #, into a powerful social networking tool. The symbol is the basis of a hashtag, a way for users to organize Twitter messages (tweets) around certain topics — and potentially increase a topic's popularity.
You make a hashtag simply by adding # to the start of a word or phrase. To hashtag this article, for example, you could write "#whatisahashtag."
Hashtags act as labels, and Twitter users can search with hashtags to find people talking about a particular subject. Entering a hashtag into the search window on Twitter produces a live feed of all tweets containing that hashtag. You can also just click on an instance of a hashtag, since Twitter automatically makes each one a hyperlink.
The first suggestion of # labeling in Twitter appeared in an August 2007 tweet by Chris Messina, an open-source software advocate. However, the tagging method actually dates back to Internet Relay Chat, a chatting protocol created in 1988.
Widely used hashtags can earn a spot on Twitter's "Trending Topics" board, which appears on the lower left of a Twitter screen in an Internet browser — potentially earning those topics even more attention.
Hashtags also appear on Instagram, Flickr, Google+ and other social networks. Facebook added hashtags in early 2013.