An I/O interface is a means of communication between two sets of devices – the device under consideration and the outside world. Since, there is a communication going on, there must be some set of rules to govern the communication. Both the devices may act as transmitter and receiver. Data sent by one device is received by the other, and vice-versa. This set of rules, which governs the transmission and reception of signals, is what we term as an I/O interface.