Twitter is a social media platform launched in 2006 that allows users to post short messages called tweets that are limited to 140 characters. It grew to be one of the top 10 most visited websites and allows users to publicly share information quickly, while also allowing for private messaging between users. The document outlines various Twitter terminology and conventions for composing and sharing tweets, as well as how to engage with other users through features like retweets, mentions, hashtags, and following accounts.