Twitter is a social networking and microblogging site which enables its users to send and read other users' messages called tweets. Tweets are text-based posts of up to 140 characters displayed on the user'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. 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. Registration required.