I've seen several different descriptors for disc brake pads: Metallic, Semi-Metallic, and Organic. What is the difference between these types of disc brake pads?
Youll run into two types of metallic pads- sintered metallic and semi-metallic. The term "metallic" without any modifier gets used for a couple of reasons- a) many manufacturers don't offer both varieties of metallic pads for a given brake, so the type is implicit, and b) lots of folks don't know the difference or think the terms are interchangeable. They are similar, but different:
Sintered metallic pads offer the best stopping power, but also cause the most rotor wear and can make the most noise, especially when wet. They are also the least affected by adverse conditions. Sintered metallic are typically used in downhill/freeride applications, but can be used for less demanding riding types as well. Depending on the setup, sintered pads can feel "grabby", that is, that they lack modulation at the lever. That problem tends to arise on more powerful brakes with larger rotors though, and both of those factors play towards that perception.
Semi-metallics are a tradeoff between braking performance and pad wear/noise. They still stop very well but can be a little less noisy and cause a little less rotor wear. They may also offer better lever modulation than sintered metallic pads. This pad type can often be found on higher end all-mountain, trail, and cross country oriented brakes, though some manufacturers opt for sintered pads while others may opt for organics.
Organic pads are the kindest to your rotors and typically quitest, but don't offer the same bite as metallic pads. These pads also wear the quickest. This does not mean that they're low end pads though, and depending on conditions, riding style and personal preference they may be a great choice. Organic pads can offer the best lever modulation of the three pad types, but may not stop riders sufficiently in demanding circumstances.