In the pre-Twitter days, # was known as a pound or number sign, the lowliest key on a telephone's numeric pad. Then, in 2007, Twitter placed the pound sign in front of a targeted word to identify tweets on a specific topic—and the hashtag made its Twitter debut.

Its arrival on the social platform didn't lead to automatic acceptance from Twitter users, however. Folks first had to understand what a hashtag was and how to use it.

Hashtag use finally took off in 2009, when Twitter started automatically linking anything preceded by the pound sign.

Since then, the hashtag's popularity has continued to grow. Today, tweets that contain hashtags are 55% more likely to be retweeted than tweets that don't have them. And now other social networks, such as Instagram, Vine, and Google+, have adopted the hashtag.