Hashtags are 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! So what are they used for?
Hashtags are everywhere. Politicians have them (#Obama), sport leagues (#Superbowl, #NBA) and television shows like "American Idol" (#idol). 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.