Computer peripheral devices have different operating characteristics. When a device is connected to a computer, an interface is the hardware and software needed for the processor to be able to communicate with the device.

There are a number of different processors (and new ones being developed all the time) so it is important to adopt standards so that if the processor changes then the device can still communicate.

Sometimes the interface hardware comes on an interface board, which has to be fitted into a computer.

Sometimes the device can just be connected to a socket. This is the port through which signals to and from the device travel. 

Most PCs come with at least a serial port (for mouse or modem) which is being replaced by a USB (Universal Serial Bus) port, and a parallel port (for printers).

When two devices need to communicate, one device sends a message to the other ('Are you ready to receive data?'). When the other is ready, it sends a signal back to the first device ('Yes, I am ready to receive data')..., and then communication can begin. This exchange of preliminary signals is called handshaking.