A hash tag
is a way of organizing content, trending topics, and conversations on Twitter
. Clicking on a hash tag will show Twitter users at a glance all the Twitter conversation on that particular topic; conversely, appending a hash tag to the end of a Twitter post (known as a "tweet") will add that tweet to the ongoing Twitter conversation about that topic. To create a hash tag of your own, just type the pound sign (#) next to a topic.
One of the most popular hash tags on Twitter is "#followfriday". This tag encourages Twitter followers to follow particular Twitter users recommended by the Twitter user appending the #followfriday hash tag to a tweet; i.e., @mercycorps @nytimes @ideagov #followfriday.
Examples of Twitter hash tags: