Here is too much information:
Class Features
Class 8.8 Made of medium-strength steel.
Class 10.9 Made of high-strength steel.
Class 12.9 Made of Class 12.9 steel for increased wear resistance.
Finish/Coating Features
Plain Good for general purpose applications.
Zinc-Plated Provides excellent corrosion resistance.
Cadmium-Plated Offers better rust resistance than zinc-plating, especially in salt environments.
Nickel-Chrome Plated Polished and buffed to a bright, mirror-like finish. Resists wear and corrosion.
Black-Oxide Offers mild rust resistance and some lubrication qualities.
Blue-Coated This highly visible blue coating makes it easier to distinguish from inch sizes.
Ultra Corrosion-Resistant Coated Also known as armor coat. Provides better corrosion resistance than zinc, cadmium, and hot-dipped galvanized plating. The thickness of the coating does not interfere with the thread fit.
Material Type Features
Plain Steel Good for general purpose applications.
18-8 Stainless Steel Provides excellent corrosion resistance. May be mildly magnetic.
300 Series Stainless Steel Meet more stringent specifications such as military specifications. Corrosion Resistant.
316 Stainless Steel Offers excellent corrosion resistance, even more than 18-8 stainless steel. Contains molybdenum which increases corrosion resistance to chlorides and phosphates.
Bumax 88 Stainless Steel 316L stainless steel with a high molybdenum content offering corrosion resistance similar to 316 stainless steel. May be mildly magnetic.
Brass Nonmagnetic and softer than stainless steel and mild steel.
Nylon 6/6 Nonconductive and resistant to chemicals and solvents (except mineral acids). Since nylon absorbs moisture from the environment, changes in moisture content will affect the fastener's dimensions and properties. Withstands a wide range of temperatures.
Silicon Bronze Made of 95-98% copper with a small amount of silicon for strength. Nonmagnetic and offers high thermal conductivity and corrosion resistance.
A286 Super Alloy Made of 26% nickel and 15% chrome with corrosion resistance similar to 18-8 stainless steel and strength properties comparable to alloy steel. Is considered an iron-based super alloy. Passivated (a nitric acid treatment that creates a passive film to protect against oxidation and corrosion).
For a better description and comparison of materials:
Mcmasters scroll all the way down on the right look for raw material then metals select the material that you want to compare and see for yourself. Aluminum vs. stainless vs. titanium vs. carbon steel. Hmmm...interesting ;)