Marc Abramowitz has published a quick and dirty Linux User Mode home.
User-mode Linux (UML) allows multiple virtual Linux systems o run as an application within a normal Linux system. Each guest is just a normal application running as a process in user space.
From the article:
User Mode Linux (UML) allows you to run Linux kernels as user mode processes under a host Linux kernel, giving you a simple way to run several independent virtual machines on a single piece of physical hardware. Let's take a look at UML and how it can give you more bang for the hardware buck, or make it easier to debug the kernel.
Under UML, each of the virtual machines can run its own selection of software, including different distributions of Linux and different kernels. This gives you the ability to have completely customizable virtual machines that are isolated from each other, and from the host machine. Among other things, you can use this technology to secure systems by containing vulnerabilities, to give developers and sysadmins private sandboxes for development and testing, and to debug problems in kernels using familiar userspace utilities, such as gdb.