Twitter is a free social networking and micro-blogging service that enables its users to send and read each others' updates, known as tweets. Tweets are text-based posts of up to 140 characters, displayed on the author's profile page and delivered to other users - known as followers - who have subscribed to them. 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. The service is free over the Internet, but using 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", as it provides the functionality—via its application programming interface (API)—for other desktop and web-based applications to send and receive short text messages, often obscuring the Twitter service itself.