Traditionally, an emulator is software that translates programs written to run on one instruction set so they can run on another one. The classic example is Apple Computer's migration of the MacOS from the 68k architecture to the PowerPC. It turned out to be easier to port the only critical parts of the operating system and use an emulator for the rest. In general, emulators tend to degrade performance because it takes time to do the translation.
There are two popular emulators available that allow you to run Windows inside of Linux: VMware and Win4Lin. As with any emulator, you should not expect full performance, but it is important to remember that these emulators are not translating any instructions since both Windows and Linux running on a PC use the same x86 instruction set. What Windows emulators do is create a "virtual PC" that you can use to install a genuine copy of Windows. (Of course, this means that you have to own a copy of Windows to use either of these products.)
Vmware takes a more complete approach that allows you to install several operating systems into what are called "virtual machines." Using VMware, you can run Linux applications inside Windows or you can run Windows applications inside Linux, and the same is possible for other operating systems. Win4Lin, from NeTraverse , was designed from the ground up for a different purpose: to enable Linux users to run Windows applications.
Win4Lin has several advantages over VMware. First of all, Win4Lin has a much smaller memory requirement. With VMware, I was unable to get decent performance with less than 128 MB of RAM, but Win4Lin works fine in a computer with only 64 MB. The second advantage is that Win4Lin allows you to use a single file system for both Windows and Linux. With VMware you still need to use two separate file systems, just as in the dual boot, but with Win4Lin, the Linux file system is used for both operating systems.
Win4Lin supports the latest versions of Red Hat, Caldera, SuSE, and Mandrake, but you may have a wait a few weeks when a distribution releases a new version. You should also be able to use Win4Lin with the "official" Linux kernel at kernel.org . If you go this route, you will also need to get the kernel patch, which can be downloaded for free from NeTraverse.
The download version of Win4Lin is $79.99 and the CD boxset is $89.99. If you purchase a license over the web, NeTraverse will email it to you.