The receiver is what you place in the car to receive the radio signals.

A Module is a RF device that attaches to the radio transmitter to send the signals to the receiver. Not all radios require a module because many if not most have the module built in.

You only need a module if your radio requires one example: Futaba 3PK/PKS, Airtronics M8/M11, etc.

And yes if your radio needs a module then you need both a module and a matching band receiver, i.e.; 75MHz FM, 27MHz FM, 75 MHz PCM, 75 MHz HRS, 2.4 GHz DSM, etc. These are examples of bands.