Player pianos (usually known as pianolas, though that was originally a trade name) are normally operated by air (actually suction rather than pressure); they are a conventional piano (upright or grand) which incorporates a mechanism which plays the notes under the command of holes in a perforated paper roll.  They are typically powered using pedal-operated bellows, by which means the operator (sometimes called the pianolist) can vary the loudness of the music by pumping with different intensities.  Further expression can be added using the hand controls which control functions such as tempo, damping (the "sustain pedal" of a piano, noting that the conventional pedal isn't accessible when pumping!), balance between treble and bass etc. A description of how the pianola reads the notes from the roll is given here, and some audio and video clips of pianolas playing standard rolls are given here.