Modern Computer System.
Modern, universal computer system consists of central processor unit (CPU) and few device drivers for attached hardware, joined with common bus enabling them to contact shared memory.
Each of device drivers is responsible for given type of hardware (for example: hard disks, sound cards, displays, ...). They abstract over it, enabling users to communicate with multiple types of hardware, for differences are handled by device drivers.
There can be 'plugin drivers' for different type of hardware, that cooperate with device driver that user processes talk with. Processes are running programs, so device drivers are programs that let users 'talk with' and control hardware.
Central processing unit and device drivers can work concurrently (simultaneously) competing for resources such as processor cycles and memory space and access time. Memory driver has to provide for ordered, synchronized access to shared memory.
See also: Hardware, Operating System and Computer System (for everyone).