In automotive electronics, Body Control Module is a generic term for an Electronic control unit

Electronic control unit

In automotive electronics, electronic control unit is a generic term for any embedded system that controls one or more of the electrical systems or subsystems in a motor vehicle....

 responsible for monitoring and controlling various electronic accessories in a vehicle's body.
Typically in a car the BCM controls the Power windows, Power mirrors, Air conditioning

Air conditioning

An air conditioner is a home appliance, system, or mechanism designed to dehumidify and extract heat from an area. The cooling is done using a simple refrigeration cycle...

, immobilizer system, Central locking, etc.
The BCM communicates with other on-board computers via the car's Vehicle bus

Vehicle bus

A vehicle bus is a specialized internal communications network that interconnects components inside a vehicle...

, and it's main application is controlling load drivers - actuating relay

Relay

A relay is an electrically operated switch. Many relays use an electromagnet to operate a switching mechanism mechanically, but other operating principles are also used. Relays are used where it is necessary to control a circuit by a low-power signal , or where several circuits must be controlled...

s that in turn perform actions in the vehicle such as locking the doors or dimming the salon overhead lamp.