QEMU is a generic and open source machine emulator and virtualizer. When used as a machine emulator, QEMU can run OSes and programs made for one machine (e.g. an ARM board) on a different machine (e.g. your own PC). By using dynamic translation, it achieves very good performances.
Virtualization is the technique of running a "guest" operating system inside an already-running OS; for example, Windows inside Linux, or visa-versa. This week Canonical, the company behind Ubuntu Linux, announced a partnership with Parallels, maker of the Virtualization products Parallels Workstation and Parallels Desktop for Mac. This article compares four virtualization products available for Ubuntu Linux: the free, open source Qemu; the closed-but-free versions of VirtualBox and VMware-Server, and the newly-available, commercial Parallels.