Selected Wikipedia definition :
Twitter is a free social networking and micro-blogging service that enables its users to send and read other users' updates known as tweets. Tweets are text-based posts of up to 140 characters in length which are displayed on the user's profile page and delivered to other users who have subscribed to them (known as followers). Senders can restrict delivery to those in their circle of friends or, by default, allow anybody to access them. Users can send and receive tweets via the Twitter website, Short Message Service (SMS) or external applications. The service is free to use over the Internet, but using SMS may incur phone service provider fees.
Four gateway numbers are currently available for SMS — short codes for the United States, Canada, India, and a United Kingdom-based number for international use. There is also a short code for Vodafone users from the UK. Several third parties offer posting and receiving updates via e-mail.
There are many services and applications that work with or enhance Twitter. Many are designed to allow easy access to Twitter from specific devices, such as the iPhone or BlackBerrywhilst others are designed to make it easier for users to access and update their Twitter account.