Twitter uses hashtags to identify trends and unify users with similar ideas and interests. Anyone can invent a hashtag at any time by putting words together behind a pound sign. If you want to see a hashtag's history before you use it, use the Twitter search function to see all public tweets that use the tag.

Twitter's Search Function

The search function allows you to find both users and tweets on Twitter. To do a search for tweets that include a specific tag, enter the tag into the box with the pound sign included -- for example, #BobRoss -- and then press "Enter." If users have been using that tag from a public account, you will see the results. If the tag has never been used before, you will not see any tweets -- instead, Twitter will return the result, "No Tweet results for #BobRoss."