A Bot Herder is a person who controls a botnet. Using a variety of methods that are becoming more complex as time goes on, Bot Herders can issue orders to tens of thousands of "Zombies" (computers infected with spambots). A lot of botnets are controlled from IRC channels that the malicious software automatically connects to when an Internet connection is established.
Bot Herders can make a considerable amount of money by selling the botnet to another person or simply spreading software like scareware or other forms of malware for a fee. Bot Herders are responsible for potentially billions of spam e-mails and instant messages from botnets like Waledac.