Noise Cancelling Headphones are designed to block out ambient background noise that can interfere with listening experiences. There are a few different types of noise cancellation: passive, active (aka ANC), adaptive and adjustable Noise Cancellation.
Headphones with Passive Noise Cancelling use the seal and design of the earcups or earbuds as physical barriers to dampen and block out external sounds. All other types of Noise Cancelling Headphones take advantage of Passive Noise Cancelling designs in addition to their other technologies.
Some such as Sony, Bose and JBL, utilise Active Noise Cancellation to take advantage of built-in microphones to detect the frequency of background noise, emitting an inverse soundwave that cancels out the noise.
Adjustable Noise Cancellation Headphones can manually change their levels of Noise Cancellation to suit the situation, while Adaptive Noise Cancellation adjusts levels automatically in real time for optimal results as found in some Sennheiser Headphones.