Zombie

A zombie computer is a computer that has been infected by a virus and has become a “zombie”.  The zombie computer acts as a remote for the virus and will do what the virus tells it to, such as send out the virus via emails.    Essentially your computer has been hijacked and the hacker is using the resources on your computer.  You may still have access to the computer and may not even know that your computer is a zombie.  

Source URL: 
http://wikipedia.org
http://computer.howstuffworks.com