Twitter is a microblogging service that allows users to post short messages called tweets that are limited to 140 characters. Tweets can include text, links, and status updates about what a user is doing, like drinking coffee or speaking at an event. Users can choose to make their tweets publicly visible or only share with approved followers. The document discusses how Twitter can be used to keep friends updated, share links and breaking news, and describes it as a mix between blogging and chatting.