Communication between a user-mode application and the driver that drives the hardware, is implemented differently for each operating system, using the the custom OS Application Programming Interfaces (APIs).

On Windows, Windows CE, and Linux, the application can use the OS file-access API to open a handle to the driver (e.g., using the Windows CreateFile() function or using the Linux open() function), and then read and write from/to the device by passing the handle to the relevant OS file-access functions (e.g., the Windows ReadFile() and WriteFile() functions, or the Linux read() and write() functions). On Mac OS X, it's not possible to use file-access APIs to directly access the hardware from a user-mode application.

On Windows, Windows CE, Mac OS X, and Linux, the application sends requests to the driver via I/O control (IOCTL) calls, using the custom OS APIs provided for this purpose (e.g., the Windows DeviceIoControl() function, the Mac OS X IoConnectCallMethod() method, or the Linux ioctl() function). On Mac OS X, the driver also sends requests to the application via IOCTL calls.
The data passed between the driver and the application via the IOCTL calls is encapsulated using custom OS mechanisms. For example, on Windows the data is passed via an I/O Request Packet (IRP) structure, and is encapsulated by the I/O Manager.