A hashtag is a word or acronym used to describe a tweet in order for people to easily follow the conversation. Hashtags began as a community effort to tag individual tweets. Once their popularity took off, Twitter began supporting hashtags by automatically linking them into the search engine.
How do I use a hashtag?
Tagging your message on Twitter is easy. Simply add a hash (#) in front of the word, phrase or acronym that best describes your message. Due to the limited number of characters allowed in a tweet, the best hashtags are usually a single word or a few letters.
Hashtags do not support spaces, so if you are using two or more words as a hashtag, remember to skip the spaces. For example, hashtags for following the Iran election are tagged as "#iranelection".
Find the definitions to more terms in the Twitter Glossary