A bot is an automated Discord account. It utilises the Discord API. They have a "BOT" tag next to their username. They can be added through the API. Bots typically follow a command structure, where a user sends a prefixed message and the bot reponds, though bots can work in many different ways.
As of April 2020, there are over 3 million bots created to date, and they send over 28.5 billion messages each year.