A "Skin" is the material (cloth, leather, or vinyl) that you actually see when you look at the seat of a vehicle. Most can be easily replaced using simple hand tools. Sometimes a set of hog-ring pliers will be necessary, depending on the application.
Aftermarket skins don't always fit perfectly, and it's very common for the grains and/or colors to not match either. Unless you're replacing all of your upholstery at once, we heavily advise against using aftermarket skins. Replacing one single piece (i.e. Drivers Seat Bottom) with aftermarket leather will almost always be more trouble than it's worth. This is exactly why the good aftermarket leather companies (Like Katzkin) only sell interiors as a set, you can't just order parts.