As its name suggests, a noise reduction pedal (also known as a noise gate or noise supressor) isn't really an effect but a way to reduce the noise created by noisy guitars, amplifiers, or other effects (especially compressors and overdrive/distortion effects). A noise gate will allow sound through the pedal only after it reaches a certain level. Anything below the specified level, and it will "close the gate" on the sound. Of course, you have be careful not to eliminate any of the music you're trying to create, especially with sustained sounds. Noise gates can also be used creatively with reverbs to create gated drum effects (just ask Phil Collins). Noise reduction pedals have come a long way in the past few years so that they can exist harmoniously with other pedal effects without causing undue signal loss.