From WSU Technology Knowledge Base
The # symbol, called a hashtag, is used to mark keywords or topics in a Tweet and Facebook/Instagram post or comment. 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.
- What Are Hashtags? from the Twitter Help Center