Device drivers provide an interface between operating systems and hardware devices. They are split into character, block, and network modules. Character drivers handle streams of bytes like files and implement open, close, read, and write functions. Block drivers are similar but differ in how the kernel manages data internally. Network drivers send and receive data supervised by the device subsystem. Device drivers provide mechanisms and not policies, and security is enforced by kernel code to avoid encoding policies directly into drivers. Loadable modules can add functionality while the system is running.