Because Twitter provided no easy way to group tweets or add extra data, the Twitter community came up with their own way: hashtags. A hashtag is similar to other web tags because it helps add tweets to a category.
Hashtags have the 'hash' or 'pound' symbol (#) preceding the tag, like so: #traffic, #followfriday, #hashtag. Hashtags can occur anywhere in the tweet, but Haiku hashtags, ever so popular on Twitter for example, follow a set netiquette and list the hashtags at the end. Although hashtags aren't considered an official feature, they are widely popular as visible on Twitter Search (which usually has a hashtag term in Trending Topics).
If you add a hashtag to your tweet and you have a public account, anyone who does a search for that hashtag may find your tweet. Although any keyword with a # in front could be considered a hashtag, there are many commonly used hashtags. Aside from not using hashtags for spam purposes, there are no formal rules for hashtag usage. Twitter recommends a couple of best practices: Only use hashtags on tweets relevant to the topic, and do not over-tag a single tweet. To learn more about hashtags and Twitter, read the article below. It's a strange new twitterverse out there <(-'.'-)>