There are 7,500 parts in every piano, and these parts work together to form a sound unlike any other instrument – a sound produced by both string and percussion elements. The key is the short end of a much longer lever. When you press the key, the lever moves like a seesaw, forcing a small, felt-covered hammer to hit the piano strings. The vibration of the strings forms the musical note. Just behind the hammer is the damper. When the key is pressed, the damper is forced upward. When the key is released, the damper comes to rest in contact with the strings, stopping the vibration and ending the note.

Source: Liverpool Academy of Music