A botnet is network of computers infected with malware that can be used by a hacker to do their bidding.
For a hacker, using a botnet is cheaper and less vulnerable to being shut down than renting dedicated server space. Tricking users into installing malware allows the attacker to steal processor cycles, and often leaves the user unaware that their machine is infected.
Botnets are frequently used to commit denial of service attacks, send spam, or rig search engine results. Having software agents operating across of a variety of IP addresses is often vital to the attacker’s purpose; malicious traffic coming from many different sources is much harder to blacklist.