In order for microcontrollers to work effectively they need to be able to communicate with devices without needing to be connected to a host computer. This is important otherwise the micro part would be lost; in order to accommodate the computer controllers would have to be far larger and more expensive.
What is CAN bus? Officially, CAN is a Controller Area Network, which is a network of independent controllers communicating securely. It was first developed by Bosch and Intel in 1990 and has been amended since. The International Standards Organisation (ISO) has further defined CAN using their OSI model.