Take any vehicle that is intended to be driven, and it would be unmanageable without properly functioning brakes fitted inside it. These days you typically have disc brakes in most modern cars, which comprise brake pads that wear down with use. How often these need to be replaced, is one of the factors on which to base your choice of brake pad type. You can get a semi-metallic, metallic, or ceramic brake pad based on your driving style and braking needs. Following is a look at the ceramic and semi-metallic types differ from each other.
Semi-metallic pads are made up of multiple metal bits, as well as bonding material which holds these as one, and imbues the brake pad with the intended structure. The metals used here can range from iron dust and metal wire to steel wool. You might also find a copper and graphite in the mix. Semi-metallic pads generally comprise somewhere around 30 to 65 percent metal, with some "low-metallic pads" containing even less metal than that. Ceramic brake pads have a much simpler construction, consisting of ceramic fibers for the most part, and sometimes also a small quantity of metal.
Both pad types are put to the same kind of use after being inserted into the braking system. They are attached to the movable caliper, in a way that when the latter is activated by brake pedal action, the pad moves in and presses down on the spinning rotor disc. That contact produces friction, which slows and sometimes stops the car. Brake pads wear down over time and form brake dust, which either spreads out over the inner side of the wheel or simply falls onto the road when the car moves. Pads that have sufficiently worn down would need to be replaced, whether your car uses the ceramic type of pad or the semi-metallic.
Each brake pad type has advantages over the other. Semi-metallic pads cost less and deliver better heat dispersion in the direction opposite to the rotor. The latter is protected from warping as a result. Semi-metallic pads are also much more durable and take longer to wear down enough that they require replacing. Ceramic brake pads, in comparison, are much softer, but this means they apply less pressure on the brake rotor, which in turn last for a longer time. Ceramic pads are also a lot smoother and quieter when the brakes are applied, which means vehicle stopping is very consistent.