Noise-canceling headphones reduce unwanted ambient sounds from outside the headphones using active noise control. This is distinct from passive headphones which, if they reduce ambient sounds, do so via physical characteristics such as a closed-back or sealed design.
The goal of noise cancelling headphones is to allow the user to listen to source material in loud environments — such as on airplanes — and/or to improve listening to offset the effect of a distracting concurrent external activity.