WOW is an acronym for Windows-on-Windows.
Windows NT provides a combined Virtual DOS Machine and WOW environment. Because Win16 programs were made to interact with each other, each additional Win16 application is loaded into the same VDM and WOW that was established for the first Win16 application that was started. In Windows NT, applications can cause each other to fail, but only the VDM will fail, not the operating system.
- WOW Input Queue
Win3.1 and earlier had only one input queue for all user interface requests. Win16 recreates this environment, so each VDM and WOW pair has one input queue for all Win16 applications in that particular VDM/WOW pair. Running applications can interfere with each other.
- Scheduling in WOW
Win3.1 and earlier distributed execution time with cooperative multitasking. Win16 within a VDM/WOW multitasks cooperatively. In NT, the applications run cooperatively within the VDM/WOW pair but multitask preemptively between the VDM and the rest of the 32-bit applications.
Additionally, lj says: You might want to mention the 'run in a separate memory space' checkbox, which causes NTs to start a new wow for the process. It seems to have disappeared in XP, though.
Updated to add: WOW is also an acronym for the video game World of Warcraft.