botnet

a network of anywhere from thousands to millions of computers around the world that can be unknowingly controlled by a single hacker (known as a "bot herder" or "bot master"). Botnets of zombie computers are often used to spread e-mail spam and launch denial-of-service attacks. Botnets can be purchased or rented, as reported by the Wall Street Journal in 2012, for as little as $2/hour or $700 to buy.