A zombie is a malicious program which is inserted onto a machine (often via a virus or worm) and then allows a remote user to control that machine. These machines are often used in denial-of-service and distributed denial-of-service attacks.

A typical zombie is installed when a virus is executed by someone opening an infected attachment to an email message. The zombie immediately installs itself, then sends out a message to let people (hackers) know that it is waiting for commands. Hackers can then command these zombies (often using IRC) to do various things, including updating themselves with modules to perform different functions.

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Jargon dictionary entry
Hacker Notes - Zombies Sometimes a virus does not appear to do anything, yet has infected your system. If this happens, you might actually be helping attack a government, a company or a university.