Tweets are publicly visible by default; however, senders can restrict message delivery to just their followers. Users can tweet via the Twitter website, compatible external applications (such as for smartphones), or by Short Message Service (SMS) available in certain countries. While the service is free, accessing it through SMS may incur phone service provider fees.
Users may subscribe to other users' tweets – this is known as following and subscribers are known as followers or tweeps (Twitter + peeps). The users can also check the people who are un-subscribing them on Twitter better known as unfollowing via various services.
Twitter allows users the ability to update their profile by using their mobile phone either by text messaging or by apps released for certain smartphones and tablets.
Twitter has been compared to a web-based Internet Relay Chat (IRC) client. In a 2009 Time essay, technology author Steven Johnson described the basic mechanics of Twitter as "remarkably simple":
As a social network, Twitter revolves around the principle of followers. When you choose to follow another Twitter user, that user's tweets appear in reverse chronological order on your main Twitter page. If you follow 20 people, you'll see a mix of tweets scrolling down the page: breakfast-cereal updates, interesting new links, music recommendations, even musings on the future of education.