According to hashtags.org “hashtags” are a community-driven convention for adding additional context and metadata to your tweets. They’re like tags on Flickr, only added inline to your post. You create a hashtag simply by prefixing a word with a hash symbol: #hashtag.
Hashtags are getting more and more popular (despite the fact some people are against them wishing their Twitter stream to be metadata-free and “as human a form of communication as possible”). Using the sign # before a word, means that you want to talk about a particular thing (event, individual or place) on Twitter.
This is the way to tag a conversation so everyone can filter it using Twitter Search.
#hashtags.org is the first place to find most recent and hot hashtags. It also shows each hashtag popularity trend when you hover over.
These are some popular Twitter Hashtags:
Here are some top hashtags from hashtag.org