Twitter is a microblogging platform that allows users to post short updates called tweets. It can be used as a marketing and PR tool to connect with bloggers, alert customers to deals, and promote content. Twitter can also help with job hunting by building a professional network, using hashtags, and connecting with potential employers. Setting up an account involves choosing a username, profile photo, and bio and learning how to compose tweets within the 140 character limit using hashtags and @mentions.