I looked up some info on WMF's and EMF's once. From what I recall WMF
(Windows meta file) is a 16-bit version and EMF (enhanced meta file)
is the 32-bit version. So use EMF where possible.

The "meta" in the name is because the file format contains both vector
and raster information in the file. That allows you to resize them and
still have the image and text render nicely. It's a nice format for
working with pictures within Office apps.

[Disclaimer: I'm working off a fuzzy memory here, so I may not be 100%
correct, but I'm pretty sure I got the gist of it.]

Nicholas Hebb
BreezeTree Software
http://www.breezetree.com