Twitter is a microblogging platform that allows users to post messages of up to 140 characters. It can be used for marketing, branding, and sharing information. To use Twitter, one signs up, builds a profile, follows others with similar interests, and posts regular messages while avoiding overly personal content like breakfast details. The "@" symbol is used to mention other users, "D" sends a direct message, and tools exist to search tweets, get alerts, and enhance the Twitter experience.