Citation from wiki.qemu.org:

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 performance.

When used as a virtualizer, QEMU achieves near native performances by executing
the guest code directly on the host CPU. QEMU supports virtualization when executing
under the Xen hypervisor or using the KVM kernel module in Linux. When using KVM,
QEMU can virtualize x86, server and embedded PowerPC, and S390 guests.