What exactly is a hashtag (#)?

[Tucker]

People on Facebook/ etc. have always been doing #(whatever) and I don't understand what it does/means. Please help.

[Not A CowGirl]

its like the theme of what you're talking about

"I don't want to be at school #bored"

The theme is obviously that you are bored at school.

[Mathers]

-A word or phrase preceded by a hash or pound sign (#) and used to identify messages on a specific topic.
-To put it simply, a hash tag is simply a way for people to search for tweets that have a common topic and to begin a conversation. For example, if you search on #LOST (or #Lost or #lost, because it's not case-sensitive), you'll get a list of tweets related to the TV show. What you won't get are tweets that say "I lost my wallet yesterday" because "lost" isn't preceded by the hash tag.