Twitter is a social networking and microblogging service that enables its users to send and read other user 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.

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 in July 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.

The tipping point for Twitter's popularity was the 2007 South by Southwest (SXSW) festival. During the event Twitter usage increased from 20,000 tweets per day to 60,000. "The Twitter people cleverly placed two 60-inch plasma screens in the conference hallways, exclusively streaming Twitter messages," remarked Newsweek's Steven Levy. "Hundreds of conference-goers kept tabs on each other via constant twitters. Panelists and speakers mentioned the service, and the bloggers in attendance touted it." Reaction at the festival was highly positive. Blogger Scott Beale said that Twitter "absolutely rul[ed]" SXSW. Social software researcher Danah Boyd said Twitter "own[ed]" the festival. Twitter staff received the festival's Web Award prize with the remark "we'd like to thank you in 140 characters or less. And we just did!"

By March 2010, Twitter recorded a 1,500% increase in the number of registered users and its staff had grown fivefold. Over 70,000 registered applications have been created for Twitter, according to the company.

Twitter had 500,000 tweets posted per quarter in 2007. This grew to 100 million tweets posted per quarter in 2008. By the end of 2009, 2 billion tweets per quarter were being posted. In the first quarter of 2010, 4 billion tweets have been posted. Currently, about 65 million tweets are posted each day, equalling about 750 tweets sent each second, according to Twitter. Additionally, Twitter's usage spikes during prominent events. For example, a record was set during the 2010 FIFA World Cup when fans wrote 2,940 tweets per second in the 30 second period after Japan scored against Cameroon on 14 June 2010. The record was broken again when 3,085 tweets a second were posted after the Los Angeles Lakers' victory in the 2010 NBA Finals on 17 June 2010.

Twitter acquired application developer atebits on April 11, 2010. Atebits had developed the Apple Design Award winning Twitter client Tweetie for Mac and iPhone. The application, now called "Twitter" and distributed free of charge, is the official Twitter client for the iPhone.

Technology author Steven Johnson describes the basic mechanics of Twitter as "remarkably simple:"