Twitter is a popular microblogging social network service. The San Francisco-based company was founded by Jack Dorsey, Biz Stone, and Evan Williams in 2006.
Users are able to send and read messages -- also known as tweets -- that are up to 140 characters long. Tweets are publicly visible unless a user protects his or her account so that only approved users have access to it. Users can "follow" friends and colleagues, as well as their favorite brands, news outlets and other institutions. Twitter has also become a useful marketing tool for businesses.
A username, or user's "handle," is generally a series of letters and numbers preceded by the "@" sign. For instance, Entrepreneur magazine's Twitter handle is @EntMagazine. Tweets can be tracked in groups by using a hashtag, or the "#" sign, before the word or phrase.
As of September 2011, Twitter said it has 100 million active users worldwide.