Brake pads sit at the end of the brake caliper arm and push against the rim to slow the bike down. They are either a one piece pad bolted on to the arm or a shoe with a cartridge (pad) inserted. When buying cartridge pads please ensure they are compatible with the original manufacturer as shoe shapes can be different. Pads tend to come in a medium compound for use in all weathers but softer and harder compounds are also available.
Carbon rims require dedicated softer compound brake pads as normal pads will damage the carbon and not work as well.
Cantilever brakes found on cyclo cross and touring bikes need a long arm brake pad which is not compatible with standard road brake calipers.