This article is from the Piano General Topics FAQ, by Isako Hoshino firstname.lastname@example.org with numerous contributions by others.
Bellows - A component usually consisting of two like-pieces
of wood with a cloth hinge at one end, and covered with
a rubberized cloth. One side of the bellows will have
an opening, so that when vacuum is applied, a mechanical
action occurs. Conversely, when connected to pedals and
a check valve is added, they act as a pump, lowering the
pressure in the stack.
Stack - The upper part of the player. This is the part that
plays the piano, and contains the valves, bellows,
spoolbox, and wind motor.
Spool Box - This is the area where the piano roll is
inserted, and is usually behind a set of doors.
Tracker bar - The brass bar in the middle of the spool box
that has all those holes in it. Each hole represents a
note on the keyboard. They are sequential (i.e., C C# D
D# E F F# G G# A A# B). Tubes, usually made of lead,
are connected from the back of the tracker and to the
stack. Each tube is connected to a channel in the stack
that controls a valve connected to the main vacuum
supply from the pump.
Pump - The lower part of the player. The pumping pedals are
connected to the pump. The pump usually contains the
wind motor regulation, and controls to divert the vacuum
to the stack, wind motor, and expression pneumatics.
Expression pneumatic - Since the piano's usual expression
pedals are covered up by the pump pedals, it looks as if
you cannot access them. However, there is a way to
duplicate these pedals through the use of expression
pneumatics. The piano controls are usually located
underneath the hinged key slip. Usually, there is a
button which will control the equivalent pedal function
also. In order to operate the loud pedal, simply push a
button on the control rail, and the loud expression
pneumatic will operate exactly like the loud pedal. In
addition to the loud pedal, there are usually two soft
pedal expression pneumatics.