What is Twitter?
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.
Source: Twitter on Wikipedia