1) A group of computers running a computer application controlled and manipulated solely by the owner of the software source.2) A number of internet computers infected with malicious software, such as Trojans, from a hacker. The hacker controls all the computers causing them to behave like "robots". These "robots", commonly known as "bots" or "zombies", are used by the hacker who controls the botnet to send spam email, spread viruses, attack other networks  and for various other malicious purposes.