The Windows Metafile format is the standard Windows format for storing vector and metafile graphics. It is quite powerful, as anything that can be drawn on the screen or printed on a printer can be stored in a metafile. It does, however, lack several rather rudimentary graphics primitives, such as Bézier curves and non-orthogonal ellipses.
ThumbsPlus can read and write Windows metafiles; vector graphics are also passed to and from the clipboard in this format.
Also, using OLE to load or thumbnail proprietary formats most often uses Windows Metafiles as the format for transfer of the picture of the file.