How can a computer use a printer it has never met?

A computer can use a printer it has never met because of a small program called a device driver. A driver is software that controls one particular kind of device and gives other software a standard way to talk to it. Its main job is to act as a translator between a hardware device and the programs or operating system that use it.

The translator is needed because devices speak different languages. Take two ways of sending data down a serial line: the commands for one chip are very different from the commands for a USB adapter. Yet each has its own driver, and each driver presents the same, or a similar, interface to the rest of the system. A programmer can write "send data" once without caring which gadget will receive it. Even something as familiar as drawing a pixel follows this path: an application asks the kernel, which forwards the request to the display driver, which actually plots the pixel.

Drivers are specific to both the hardware and the operating system, so each gadget needs its own matching one. Modern systems try to find it for you. In plug-and-play systems the system scans the buses, such as USB, to detect devices and then searches for the appropriate drivers.

There is a price for this power. Drivers need low-level access to hardware, so they usually run in a highly privileged environment and can cause system problems if something goes wrong.