Body Control Modules (Central Body Control Modules) are essential components of vehicle electronics. They combine several functions into a single housing and control virtually all of the vehicle’s electronic foundation, comfort, and security functions. Your vehicle consists of several functions each of them controlled by several modules like module for the air-conditioning, a module for the interior lighting, a module for the door locks, module for the headlights and many more. 

Put in simple, Body Control Module (BCM) combines all of these separate modules into one system so that they can function together rather than separately, making it easier to manufacture and troubleshoot.