I get @whoever, But whats #?

[Kagami]

Okay so I know both of those originated on twitter, But now people use them everywhere, Not being a twitter user, I'm not sure what it is. A band I know tweeted and shared it on facebook, they tweeted about their tour and put "#tour" at the end, and I figured that meant, well, he was talking about the bands tour. But then I seen someone post about being outta peanut butter and they put "#Heartbreak" at the end, so that makes me think that it meant what the person was feeling, but that wouldn't explain what the #tour was. So THEN, I seen someone taking the piss outta another person who censored their cuss word, and it was "I seen that you censored your curse there, But I still know what it means. Thats stupid. #pointless", So that makes me think that he was pointing out what it was, Pointless, Which i suppose in a way would explain all three, But I really don't get it, Can someone explain?