Twitter is a social networking and microblogging service, owned and operated by Twitter Inc., that enables its users to send and read other users' messages called tweets. Tweets are text-based posts of up to 140 characters displayed on the author's profile page. Tweets are publicly visible by default, however senders can restrict message delivery to their friends list. Users may subscribe to other author tweets—this is known as following and subscribers are known as followers. As of late 2009, users can follow lists of authors instead of following individual authors. All users can send and receive tweets via the Twitter website, compatible external applications (such as, for smartphones), or by Short Message Service (SMS) available in certain countries. While the service is free, accessing it through SMS may incur phone service provider fees. The website is based in San Bruno, California near San Francisco (where the website was first based). Twitter also has servers and offices in San Antonio, Texas and Boston, Massachusetts.

Since its creation in 2006 by Jack Dorsey, Twitter has gained notability and popularity worldwide and currently has more than 100 million users worldwide. It is sometimes described as the "SMS of the Internet." The use of Twitter's application programming interface (API) for sending and receiving SMS from other applications often dominates the direct use of Twitter.

Twitter's origins lie in a "daylong brainstorming session" that was held by board members of the podcasting company Odeo. During the meeting, Jack Dorsey introduced the idea of an individual using an SMS service to communicate with a small group, a concept partially inspired by the SMS group messaging service TXTMob.

The original project code name for the service was twttr, inspired by Flickr and the five character length of American SMS short codes. The developers initially considered "10958″ as a short code, but later changed it to "40404″ for "ease of use and memorability." Work on the project started on March 21, 2006, when Dorsey published the first Twitter message at 9:50 PM Pacific Standard Time (PST): "just setting up my twttr."

The first Twitter prototype was used as an internal service for Odeo employees and the full version was introduced publicly on July 15, 2006. In October 2006, Biz Stone, Evan Williams, Dorsey, and other members of Odeo formed Obvious Corporation and acquired Odeo and all of its assets–including Odeo.com and Twitter.com–from the investors and shareholders. Twitter spun off into its own company in April 2007.