WOW is a sort of confusing acronym that shows up every now and then when Microsoft revs a major product, like SQL Server or Windows to a newer version. This acronym is even more confusing when you discover that it means "Windows on Windows". After all, isn't Windows, um, essentially running on Windows anyway?
The confusion is lifted when you discover that WOW refers specifically to running a lower-end platform of Windows on a higher-end platform. For example, the latest version of WOW is more fully and properly titled WOW64, meaning it allows you to run 32-bit software in a 64-bit environment. Without the latest version of WOW, you wouldn't be able to run SQL Server 2000 SP4 with full support on a x64 64-bit OS. Going back in time, the original version of WOW allowed you to run 16-bit applications on 32-bit Windows.
You can find out more about WOW64 at http://en.wikipedia.org/wiki/WoW64.
(Thanks to Bruce Loehle-Conger, David Gugick, and Roger Wolter for unearthing this tip!