The window mount, classically referred to as a 'passe-partout', is comprised of an aperture cut into a top mount or mat (just like a window), and an undermount which provides physical and environmental protection. It provides a border around the visible image and keeps a gap between the artwork and the glazing.
Float mounting is where all the edges of the art print or picture are exposed and then the artwork is separated from the glass with a spacer. The art appears as if it were 'floating' in the frame because you can't see it mounted to anything. The floating style of framing is popular among museums and galleries because it allows the art to breathe and not be pressed directly up against the glass.