Messina’s suggestion to use the hashtag was not adopted by Twitter, but the practice took off after hashtags were widely used in tweets relating to the 2007 San Diego forest fires in Southern California.
According to Messina, he suggested use of the hashtag to make it easy for "lay" users to search for content and find specific relevant updates; they were for people who do not have the technological knowledge to navigate the site.
A hashtag is a type of metadata tag used on social networks such as Twitter and other microblogging services, allowing users to apply dynamic, user-generated tagging that makes it possible for others to easily find messages with a specific theme or content; it allows easy, informal markup of folk taxonomy without need of any formal taxonomy or markup language.
Users create and use hashtags by placing the number sign or pound sign (also known as the hash character) in front of a string of alphanumeric characters, usually a word or unspaced phrase, in or at the end of a message.
Twitter began to hyperlink all hashtags in tweets to Twitter search results for the hashtagged word (and for the standard spelling of commonly misspelled words).
In 2010, Twitter introduced "Trending Topics" on the Twitter front page, displaying hashtags that are rapidly becoming popular.
In many cases other characters are also allowed, in particular accented characters used in many languages, but handling may vary from one client to another, and from time to time as standards evolve.
The US pound sign, number sign or hash symbol "#" is often used in information technology to highlight a special meaning.
It is considered acceptable to tag a post once when contributing to a specific conversation.
Two hashtags are considered acceptable when adding a location to the conversation.
The trending topics can be organized by geographic area or by all of Twitter.
Hashtags are neither registered nor controlled by any one user or group of users.