Also referred to as resin or semi-metallic brake pads. Organic brake pads are made from a mixture of fibers held together with a resin. Some of the materials used in organic brake pads can vary from kevlar, carbon, and rubber, among other things depending on the application.
Organic brake pads are made from softer materials than sintered brake pads meaning they are usually quieter. In mountain bike applications, organic pads will give you more initial bite when you first grab the brake lever. One of the negatives of organic brake pads is that they don't manage heat and fade long distances. Those long descents are when you need to lean on your brakes the most, and organic pads may lose some of their power when heat soaked.
The biggest downside to organic brake pads is that they do not perform well in wet conditions. In wet and muddy conditions, the brake pads can wear down faster than normal conditions and can also get glazed over, keeping them from performing again in dry conditions.
Sintered Brake pads:
Also referred to as metal sintered or metallic brake pads, sintered brake pads are made from metallic particles fused at a high temperature and pressure. Sintered brake pads are used in most OEM applications, not only in the mountain bike industry but also on motorcycles and cars, because of their ability to stop in all conditions. Sintered brake pads will continue to grab as strong as ever in wet and muddy trail conditions.
For those riders racing, sintered brake pads may work better because of their ability to be less affected by heat build-up. The heavier you are on brakes, the more heat is generated. Sintered brake pads will not fade as much as organic brake pads on those long steep descents.
The negatives of using sintered brake pads are that they can make some noise if they are wet or have been really hot. Sintered brake pads can be rather loud! Sintered pads are made from a harder material, and they can be harder on rotors. This usually isn't a problem for most riders because it takes some serious abuse to burn up brake rotors.