A device driver is a special code that interfaces a physical device into the system and exports it to the user-space processes using a well-defined API. In a UNIX-like OS, where everything is a file, the physical device is represented as a file. Then, the device driver implements all the system calls a process can do on a file.
Tip
The difference between a normal C function and a system call is just the fact that the latter is mainly executed into the kernel while a function executes into the user space only. For example, printf() is a function while write() is a system call. The latter (except for the prologue and epilogue part of a C function) executes into the kernel space, while the former executes into the user space (even if it calls write() to actually write its data to the output stream).
The system calls are used to communicate with the peripherals, with other processes, and to get access to the kernel's internal data. That's why a system call triggers a switch...