A Headphone is described as an electromagnetic transducer designed to be worn on the human head for the purpose of audio listening/monitoring, and as distinct from an earphone, or system worn in the ear. Headphones (plural) would be a pair - one for each ear. They are usually based on the principle of electromagnetic induction used to convert the electrical energy output of a headphone amplifier into acoustic energy, or sound.
There are two main distinctions regarding headphone design: open-back and closed-back, generally referred to as open and closed headphones. With closed headphones, the ear is completely sealed off from outside noise (pressure chamber principle). Typical features of closed headphones are the acoustically sealed housing and the ring-shaped (circumaural) pads that completely surround the ear. The sealing around the ear has a decisive influence on the sound reproduction of closed headphones. If it is insufficient, the quality of the bass sounds will deteriorate.