IIRC, UserMode Linux is not a distro as such. The readmes (this is from memory, BTW) say that you should compile the kernel with UML, to create a UML-kernel (I think you need to patch the kernel tree with some uml stuff), and you can then run this uml-kernel as a normal user process. So, effectively, you'd have a linux 'system' running as a process under Linux. I believe it's most beneficial for testing new kernel 'drivers' on a system that doesn't have the hardware. I could be wrong, though.