This term is used in a number of different contexts in the world of computers, audio and video production. In general, an interface is a boundary across which two systems communicate. It might be a hardware connector used to link two or more other devices, or it might be a software convention used to allow communication between two systems. The MIDI Interface is an example that uses both of these components. Remember that MIDI is an acronym for “Musical Instrument Digital Interface.” The MIDI specification defines both the hardware connections – the now-familiar 5-pin DIN connector, plus the circuitry inside a MIDI device, and the software code that provides a common language all MIDI devices understand.
With the arrival of computer-based audio recording, interfaces were developed to transmit audio (after it had undergone an A/D conversion) to the computer hardware and software. Essentially these interfaces serve to encode digital audio data into a communication protocol (for example, SCSI, USB, FireWire or proprietary formats) for transmission to a computer and translate it at that end.
A similar approach is involved when using external storage devices such as FireWire hard drives. Technically speaking, there is no such thing as a “FireWire drive.” FireWire is simply the data transmission protocol; most drives used in this context are ATA or SATA devices. They require a two-way interface that translates incoming data from the FireWire cable into a format the ATA drive can handle when writing to disk, and re-translates data read from the drive to be transmitted back over the FireWire cable.
Last but not least, the term interface is used to define the connection that allows interaction between hardware or software and a human user. The GUI, or graphic user interface, is a visual representation of the hardware or software operating system that makes operation easier (at least in most cases!). Even the small LCD or LED displays on synth modules or effects processors are examples of user interfaces.