Twitter is a social media platform that allows users to post short messages called tweets that are limited to 140 characters. It became popular due to its simplicity, open API, mobile accessibility, and timing launching at the SXSW conference in 2007. People use Twitter to share information with connections, ask and answer questions, market products/services, cover live events, and provide customer service. The document demonstrates Twitter's basic features like posting, replying, retweeting, following users, and direct messages.