User Mode Linux

User Mode Linux (UML) is an alteration to the Linux kernel that allows a complete Linux kernel to run inside another running Linux system without affecting the host environment's configuration or stability.

Numerous things become possible through the use of UML. One can run network services from a UML environment and remain totally sequestered from the main Linux system in which the UML environment runs. Often, administrators use UML to set up honeypots, which allow one to test the security of one's computers or network. UML may also serve to test and debug new software (including Linux kernels) without adversely affecting the host system.

As of Linux 2.6.0 UML has become integrated into the main kernel source tree.

"Linux on Linux", the original proposed name for UML, never saw the official light of day because the developers sensed something funny about the phrase.

External Reference

http://user-mode-linux.sourceforge.net/