QEMU is a generic and open source machine emulator and virtualizer, originally developed by Fabrice Bellard.
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.
When used as a virtualizer, QEMU achieves near native performances by executing the guest code directly on the host CPU. A host driver called the QEMU accelerator (also known as KQEMU) is needed in this case. The virtualizer mode requires that both the host and guest machine use x86 compatible processors.
Use in embedded projects
QEMU is increasingly used to provide an emulator for embedded processors, for testing embedded Linux without the need for actual hardware.
The Embedded Linux From Scratch presentation by Michael Opdenacker has great information about setting up QEMU with embedded Linux.
Also, Firmware Linux uses QEMU as part of a "native" build environment to eliminate cross-compilation problems when building for Embedded Linux distributions for non-X86 platforms.
The following architectures are supported as target architectures for system emulation: