The # symbol, is called a hashtag, which is used to describe keywords or topics in a Tweet. People on Twitter use the hashtag before relevant keywords/phrases (without spaces) in their Tweet to categorize those Tweets & help them show more easily in Twitter Search. When you add a hashtag symbol # to your tweets, your tweets can be seen by anyone via hashtag search and not just the people who follow you. So, a hashtag is an amazing way to increase your Tweet visibility.
Check out these 5 Tools To Search/Browse Twitter By Hashtag or Find the Best Hashtags to include in your Tweets:
With Tagdef, discover what different hashtags on Twitter really mean, & add your own definitions to Hashtags in seconds.
You can also search Popular, Trending, New, Top This Week and Popular Hashtags on this website.
Hashtags.org offers an amazing interface to search/browse for hashtags & their trending popularity. For example, if you search #seo hashtag, Hashtags.org will display the times of day & days of the week graphically. Apart from that, you can see latest tweets that include #seo hashtag.
Twubs are the Twitter groups built around content aggregated from #hashtags. It’s a great place to connect with like-minded people as hashtags are grouped into categories such as movies, technology, Internet, politics and many more.
Trendsmap shows the latest trends, hashtags and tweets around the world on a map.