VMware
Intel x86-compatible architectures which allows the creation and running of multiple virtual x86 computers simultaneously, which can run a number of guest operating systems, including (but not limited to) Windows, Linux, and BSD variants.
Conventional emulators like Bochs or Qemu emulate the entire PC hardware including the CPU. VMware takes a different approach, adding a thin layer of code to virtualize the real PC hardware and CPU, so that multiple OSs can run at the same time without clashing with each other.
In other words, the virtual devices an OS sees when running inside VMware are bridged straight to real PC hardware. The benefit of this approach is that an OS running inside VMware can run at almost the same speed as it would if it was the only OS on the system. The drawback, is that the OS you are running has to be compatible with your physical hardware. So unlike with an emulator, you cannot use VMware to run Mac software on a wintel PC, or vice versa.
With VMware you can try LiveCDs without burning them onto physical discs or rebooting the computer. You can also take a snapshot of an OS running under VMware, save it to a file, and roll back to that saved snapshot at any time. This makes VMware very popular as a tool for sales people to demonstrate complex software products.
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