Wine is a program that lets you run Windows programs
in Linux. Strictly speaking, it is not an emulator.
Rather, it is a Win32 interface to Linux and X window.
This means that there is minimal slow down caused by
running Windows applications in Wine.
Wine is perhaps most commonly used to run Windows
games in Linux. Indeed, a large degree of effort has
been put towards making Windows games run in Wine.
There is even a company, TransGaming, that has this as
its business model. Many popular Windows games run
without glitch, and many more run with some glitches.
Sadly, network support can be lacking in some games.
Other windows applications that have no Linux
equivalent can be run in Wine with varied degrees of
success. Apple's QuickTime viewer is perhaps the most
oft-cited example. Apple has never released a Linux
version of QuickTime, one of the more popular media
formats on the internet. In some versions of Wine, in
particular, in recent TransGaming versions, QuickTime
Player runs just fine.