Twitter is a microblogging platform that allows users to post messages called tweets that are limited to 140 characters. It has over 645 million users who post over 200 million tweets per day. The key aspects of Twitter include following other users to see their tweets, using hashtags to group tweets by topic, and retweeting or replying to other users' tweets. Proper Twitter etiquette involves being authentic, professional, and thoughtful with what you post and with who you choose to follow.