Twitter is an online social networking service that allows users to send and read text-based posts of up to 140 characters called "tweets". It was created in 2006 by Jack Dorsey and gained worldwide popularity with over 140 million active users by 2012. The document discusses how Twitter lists allow users to organize followers into groups, and provides examples of how lists can be used for media monitoring, market research, networking, and tracking competitors. It also outlines some advantages and disadvantages of using Twitter.