*Fast Lens:*
A fast lens is capable of very wide open Lens Opening and thus very low f-stop settings. It is called fast because the exposure time can be decreased when the lens is able to let in more light. Due to their wide maximum apertures, fast lenses are able to achieve very shallow depths of field.
*Slow Lens:*
A slow lens is not capable of as wide a maximum Lens Opening as a faster lens is. This says nothing about its minimum aperture (or maximum f-stop). Because it cannot open its aperture as wide, it does force a minimum depth of field that is greater than fast lenses; you cannot get as great an area to blur behind your focus plane with a slow lens as with a fast one.
