Aaron,
There are generaly two classes of decoder, Fixed and mobile; of mobile decoders there are motor control and function decoders. Motor control decoders connect to the locomotive's motor and make it go, they usually have functions control as well to turn on head ight reverse lights, ditch lights, fire box light, etc. Function only decoders are just like motor decoders but the omit the motor control circuitry and just provide the function outputs. Accessory decoders usually mount under the layout and are used to control switch machines (points are the movable rails in a turnout (switch)) or signal lights, but they can be used to urn on or off any electrical gizmo you can think of; street lights, traffic lights in town, building lights, anything! In general, when you hear someone say "Decoder" they means a mobile motor control decoder.
