A hashtag is any word in the social conversation that begins with the '#' symbol. Americans refer to it as a 'pound sign', but it's known as a 'hash' pretty much everywhere else. A '#' in front of a word like 'politics' makes it a hashtag: #politics!
What are they used for? If you haven't noticed, hashtags are everywhere! Politicians have them (#Obama), sports leagues use them (#Superbowl, #NBA), & television shows like "American Idol" (#idol) have them, too. Hashtags simply let you associate your tweets with a larger topic, group, or conversation. It's like adding keywords to your message to make it globally searchable, driving social reach and inspiring conversation.