How it works
WMF contains calls to a rendering layer that is part of Windows. This layer is called GDI, Graphics Design Interface. These system calls display the graphics on the screen. WMF is a 16-bit format introduced in Windows 3.0. It is the native vector format for Microsoft Office applications such as Word, PowerPoint, and Publisher.
The file can be compressed using ZIP and with then get the extension WMZ.
EMF, Extended Meta File, is the 32bit equivalent of WMF. Its use was extended to support printer drivers.
When Windows XP was released an Enhanced Metafile Format Plus Extensions (EMF+) format was introduced. EMF+ provides a way to serialize calls to the GDI+ API in the same way that WMF/EMF stores calls to GDI.
There are also compressed Windows Enhanced Metafile (EMZ)