A device driver is special code (running in kernel space) that interfaces a physical device to the system and exports it to the user space processes using a well-defined API, that is, by implementing some system calls on a special file. This is due to the fact that, in a Unix-like OS, everything is a file and physical devices are represented as special files (usually placed in the /dev directory), each one connected to a particular device (so, for instance, the keyboard can be a file named /dev/input0, a serial port can be a file named /dev/ttyS1, and a real-time clock can be /dev/rtc2).
We can expect that network devices belong to a particular set of devices not respecting this rule because we have no /dev/eth0 file for the eth0 interface. This is true, since network devices are the only devices class that doesn't respect this rule because network...