What is QEMU? It's a free software "processor emulator", which means it's capable of running applications written for other systems natively on your current operating system. The QEMU virtual CPU core library is released underneath the GNU LGPL. QEMU is capable of running a guest operating system on top of your current operating system, in the same manner as applications such as VirtualBox OSE and Vmware Workstation.
At its current state, QEMU is capable of launching Linux or Darwin/Mac OS X processes compiled for one CPU on another CPU; it can also boot a myriad of guest operating systems, including Linux, Microsoft Windows, DOS, Solaris, and BSD. Hardware platforms, such as x86, amd64, alpha, mips, and sparc can be emulated. QEMU can be installed from most of the Linux packaging systems.
Like VirtualBox OSE, QEMU can be a great tool for launching Windows on your current Linux desktop. If you have certain Windows applications with which you cannot part ways, QEMU can be used to start up Windows as a guest operating system on your Linux desktop; as with any guest operating system, QEMU will create a window in which that operating system will reside; the entire operating system is sustained within the QEMU launched window. Below is a screen shot of QEMU running Damn Small Linux on top of the Ubuntu Gutsy Gibbon desktop.