Rootkit
A collection of programs that enable administrator-level (root) access to a computer or computer network.
Typically, a cracker installs a rootkit on a computer after first obtaining user-level access, either by exploiting a known vulnerability or cracking a password.
Once the rootkit is installed, it allows the attacker to mask intrusion and gain control of the computer. Tools in the typical kit may include: spyware, key-logging software, and viruses to infect other computers in that network.
If a rootkit is detected on your computer, the only sure way to get rid of it is to completely erase your computer's hard drive and reinstall the operating system and all your application programs.