The #symbol is a tag added to Twitter status updates (“Tweets”) to give them additional context. To use a hashtag, simply include a keyword with a hash symbol in the body of your Tweet, e.g. #glossary .
A hashtag symbol is placed in a Tweet before the relevant keywords to categorize and improve the Tweet’s visibility in Twitter Search.
For example: #SEO is a known hashtag for search engine optimization.
The Hashtag Tools
If you are looking for hashtag trends then I suggest using tools like:
What the Trend?: The best feature of this tool is the explanation of why a particular hashtag is trending.
Tagalus: Tagalus is a hashtag dictionary and allows you to use Twitter to define and set definitions.
Hashtags.org: I really like this tool. Not only can I see the Tweets and the hashtag but also a graph that shows the activity of the hashtag.