The hashtag originated with Twitter. It's a way to connect with people around a particular keyword; for instance, if I wanted to see different perspectives or news on PRISM, I'd simply go to Twitter's search bar and type in #PRISM. This toolset aims to help you A) understand what the hashtag is and B) how you can start effectively using it.
This resource comes right from Twitter. It explains what a hashtag is and how you use it. This is pretty self-explanatory. Edit Remove Move
Making sure you're correctly using the hashtag is important. What's more important, however, is how it's used. Be user-friendly (don't make a really long hashtag if you want it to start trending) and be consistent with it. Edit Remove Move
A helpful YouTube video on how to use hashtags on Twitter. Edit Remove Move
Rachel from Social Media Examiner goes over the different ways you can effectively use hashtags for social media marketing. She dishes out some awesome tips with the best ways to utilize hashtags. But, Rachel, I must disagree with you on something -- businesses using hashtags on Facebook is a personal pet peeve of mine. Agree to disagree! Great resource, though. Edit Remove Move
This is a really cool and informative resource on how the hashtag has evolved, the different platforms it's currently on and where it's going. I like it a lot because it dives into more of the social presence of the hashtag -- for example, how people now use it to be sarcastic. Instagram's hashtag users go a little overboard, while Google+ is still doing it's own thing (while no one pays attention). Edit Remove Move