What is a zombie computer ? - Security Term

A zombie computer, or “drone,” is a computer that has been secretly compromised by hacking tools which allow a third party to control the computer and its resources remotely. When the zombie computer connects to the Internet the remote hacker can clandestinely make contact with the computer to mine data from it or use it for any number of purposes. Communication between the hacker and the computer travels through back channels of the targeted system, keeping these processes hidden from the owner.

The hacking tools used to establish hidden control in a remote computer are referred to as a rootkit. A rootkit isn’t considered malware, as there are legitimate uses for rootkits in networking. However, rootkits can also be used to target random computers on the Internet. Once a computer picks up a rootkit, it becomes an “unwilling accomplice” of the hacker, blindly following instructions, leading to the name “zombie computer.”

A hacker has full access to data and resources on a zombie computer. The infiltrator can copy, infect, corrupt or even erase the entire hard drive. He or she can also install tools that will report everything typed into the zombie computer, including usernames, passwords and financial data like credit card numbers and bank accounts. This private information can be used to commit fraud, identity theft, or can be sold or traded to others.

source: wisegeek.com