Botnet is the generic name that names any group of PC infected and controlled by an attacker remotely. Generally, a hacker or a group of them creates a botnet using malware that infects a large number of machines. Computers are part of the botnet, called "bots" or "zombies." There is not a minimum number of computers to create a botnet. Small botnets can include hundreds of infected PCs, while older botnets use millions of computers. Some examples of recent botnets are Conficker, Zeus, Waledac, Mariposa and Kelihos. Often, the botnet is understood as a unique entity, however the creators of this malware sell it to anyone who pays for it. For this reason, there are dozens of separate botnets using the same malware and operating at the same time.