qemu is a CPU emulator that runs under Linux and is designed to be relatively fast. One of the things the latest release (0.5.2) is “supposed” to do is emulate a Windows 98 environment, or rather allow you to install and use Windows 98 within the emulator and run it at a reasonable speed. It will also emulate Linux, which is useful when doing kernel development — no hard rebooting of a “real” PC to test a new kernel. It will probably do a lot of other things, but I don’t really care.
qemu, VMWare, plex86, and similar programs are system virtualization systems. They provide a “virtual computer” under which you can run a different operating system. Various I/O functions under this virtual computer are mapped to the “real” hardware. Instructions are run at nearly the same speed under the “virtual computer” as it is on the real computer because most instructions from the “virtual computer” are executed “as-is” on the real CPU. The downside to this is: the native system and the virtual system must be of the same architecture. This means I have to run an Intel-based PC to “emulate” a Intel-based PC system. For running Windows 98 under Linux (both run on Intel-based PCs), this is not a problem.
Bochs is an x86 emulator. It attempts to emulate all aspects of an Intel-based PC, not taking into account that the real computer and the virtual computer may, in fact, be very similar. While this is a much slower approach, Bochs would allow you to, say, emulate an Intel-based PC on a Macintosh.
Why am I going through this trouble, you might ask, especially when I have a boatload of real PCs in my office? For the simple reason: I don’t want to have to fire them up. Extra electricity, being one. When I travel with my Linux-based laptop, I still need to access the occasional Windows-based app that won’t run under Crossover Office. Last time I travelled, I took two laptops with me. I’d rather not have to do that again. That and it’s just a damned cool idea. If it works well enough, I might be able to replace my son’s Windows installation with Linux plus qemu with Win98 underneath it for those educational-type games.