1. Organic - Made of stuff like cellulose, which is like ground up cardboard! The cellulose is held together with a phenolic resin binder which is a heat resistant thermosetting resin. Pot handles and stuff like that is made of phenolics. Organic pads used to have asbestos to give better high temp properties but since asbestos is now a carcinogenic a no-no, kevlar, fiberglass and mineral fillers are now also used. Organic pads have a good coefficient of friction for a light pedal effort, work well at low temperatures and are very quite. They do not work so good for high performance use as they quickly wear, fade, oxidize and crumble. Organic pads are kind of old school and are common on cheap aftermarket replacement pads for older and sometimes new cars. These pads do not wear the rotors very much. Some cars have these as stock pads. Beware of these as they are worse than the stock pads. Suspect any cheap aftermarket pad. Organic pads are usually a light brown or tan in color.
2. Semi-Metallic - These have some powdered metal added to the mix to help stabilize the coefficient of friction at higher temperatures. Typically powdered Brass, iron or Bronze is added. Chopped brass or bronze wire is sometimes added to help give the pad more mechanical strength. Most stock pads on newer car are semi-metallic. Usually these pads are excellent for all-around use. They can run the gamut from very little metal to almost all metal. The more metal usually means better high temp properties, more noise, more rotor wear and less effective cold braking. Semi-Metallics run the gamut from light tan with metal flecks in them to a dark gray in color. The darker pads usually indicate a higher metal content.
3. Full metallic - These pads are made of sintered metal with very little binder. Sintered metal is powdered metal that is pressed into a mold at high temperatures until it becomes a more or less homogeneous piece. Pads of this type are pretty aggressive with ones made of brass, bronze or copper or a mix of metals being more streetable and ones using iron being more high temperature oriented. For very high temperature use, ceramic powder is added to the pad material. Axxiss Metal Matrix pads are streetable nearly full metallic pads that are made of brass and bronze powder with a resin binder. They are very streetable but I would not consider the other pads in this category as usable or even safe for street driving. Full metal pads are noisy, don’t grip when cold or wet and chew up rotors with annoying regularity. These pads make killer corrosive black brake dust so clean your rims frequently! I don’t think anyone on this list, even racers should consider anything more aggressive than the Metal Matrix or the new carbon pads. These pads are usually a dark gray to black and sometimes even copper-looking with a lustrous sheen.
4. Carbon - Carbon pads available to us mere mortals are not the amorphous carbon-carbon exotica that F-1 cars, the space shuttle and high performance jets use. They are not "carbon-fiber" either. Carbon pads that are available to us normal people are semi-metallic pads that have powdered carbon added to them to improve their high temperature properties. Personally I like most of these pads. For the most part, they have the cold friction of a good mild semi-metallic with the high temp properties of medium aggressive full metallic. Even the full race, high metal/carbon pads seem to have a fairly wide effective heat range. They for the most part are fairly good on the rotors also. Since they work so well over a broad range, carbon pads seem to have taken over the high-performance street pad market. The only drawback that these pads have is cost. They are pretty pricey. In my experience, they seem to wear a little faster than one might expect also. They also leave lots of black, corrosive, yucky, sticky, brake dust which is the main reason I will not use them on my street car. The full race carbon pads seem to eat rotors pretty well to. Carbon pads are a flat dark gray to black with a flinty look.