surface skimmers pull the film from the surface of the water. traditional protein skimmers draw from deeper in the water column.
i don't believe that surface skimmers (like this one
) actually get the proteins out of the water. If you agitate the surface (with a powerhead, etc), the film won't develop and the protein skimmer will take care of all of it.
protein skimmers also skim the film from the surface (like the Reef Octopus BH90).