Twitter is a free social networking and micro-blogging service that enables its users to send and read messages known as tweets. Tweets are text-based posts of up to 140 characters displayed on the author's profile page and delivered to the author's subscribers who are known as followers. Senders can restrict delivery to those in their circle of friends or, by default, allow open access. Users can send and receive tweets via the Twitter website, Short Message Service (SMS) or external applications. While the service costs nothing to use, 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. It is sometimes described as the "SMS of the Internet" since the use of Twitter's application programming interface for sending and receiving short text messages by other applications often eclipses the direct use of Twitter.

Twitter is ranked as one of the 50 most popular websites worldwide by Alexa's web traffic analysis. Although estimates of the number of daily users vary because the company does not release the number of active accounts, a February 2009 Compete.com blog entry ranked Twitter as the third most used social network based on their count of 6 million unique monthly visitors and 55 million monthly visits. In March 2009, a Nielsen.com blog ranked Twitter as the fastest-growing site in the Member Communities category for February 2009. Twitter had a monthly growth of 1,382 percent, Zimbio of 240 percent, followed by Facebook with an increase of 228 percent. However, only 40 percent of Twitter's users are retained.