what are # signs on trending topics in twitter for?

[phillly!]

so i use twitter, like...most of the world.haha
anyways, i understand it all, but the one thing i dont get is why
do a lot of trending topics have (#) sign's in front of them? i don't get what it's supposed to mean? anyone know?

[sillyaunt116]

The # symbol, followed by a phrase is called a hashtag. "Hashtags are a way to unite global Tweets around some particular topic".

For example, on Fridays a lot of times you will see that the #followfriday hashtag is trending. Users will tweet about interesting people to follow and then attach the #followfriday hashtag. So anyone who clicks on #followfriday in the treding topics will see the post.