Image source: Flickr

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.

• People use the hashtag symbol # before a relevant keyword or phrase (no spaces) in their Tweet to categorize those Tweets and help them show more easily in Twitter Search.
• Clicking on a hashtagged word in any message shows you all other Tweets marked with that keyword.
• Hashtags can occur anywhere in the Tweet – at the beginning, middle, or end.
• Hashtagged words that become very popular are often Trending Topics.

To note: If you Tweet with a hashtag on a public account, anyone who does a search for that hashtag may find your Tweet. 

Source: Twitter Support

Hashtags are now used by other social media including Instagram, Facebook and Flickr.

An interesting timeline of the key moments of the hashtag

Source: http://www.offerpop.com/resources/blog/history-hashtags/

#Hashtag” with Jimmy Fallon & Justin Timberlake

Good practice: Don’t #spam #with #hashtags. Don’t over-tag a single Tweet. (Twitter recommend using no more than 2 hashtags per Tweet.)

Some fun making of the overuse of the hashtag and what a conversation might sound like.