Twitter is a microblogging service created in 2006 by Jack Dorsey as a side project at Obvious, creators of podcasting service Odeo. It was formally incorporated in May 2007. Users on Twitter can post 140-character status updates ("tweets"), and get updated on tweets by other users through the Twitter website, the mobile-optimized site, SMS text messages or a large number of Twitter API-based services and clients, including clients for most mobile devices such as the iPhone.
Functionality is often developed at Twitter through a process of codifying common community practices. The syntax to reply to another user (so-called "at-replies") has been formalized from early usage, as have hashtags and 'retweeting' functionality, which allows a user to rebroadcast a Twitter message to other users.
Twitter has been extremely popular: originally the preserve of the early adopting geek market (including a lot of attendees at the SXSW Interactive conference in Austin, Texas), it is now used by a wide variety of celebrities, politicians and other famous people.