Fog is a cloud in contact with the ground. Stratus clouds are usually the only clouds that touch the ground. Fog differs from other clouds only in that fog touches the surface of the Earth. (Source and more info here).
Canon EOS DIGITAL REBEL XTi, 29/03/2008 17:50, Shutter Speed 1/125, Aperture Value 5.6, ISO 400, Lens EF70-200mm f/4L USM, Focal Length 70.0 mm