A botnet, or zombie network, is a network of computers infected with a malicious program that allows cybercriminals to control the infected machines remotely without the users knowledge. In the past ten years, botnets have evolved from small networks of a dozen PCs controlled from a single C&C (command and control center) into sophisticated distributed systems comprising millions of computers with decentralized control. Why are these enormous zombie networks created? The answer can be given in a single word: money.
Source: https://threatpost.com/economics-botnets-072209/72883/