Once you know the physical configuration of interfaces, it's time to look at the different types of interfaces available from a functional perspective (i.e., the different tasks they perform). An interface, regardless of whether it's a soundcard, breakout box with card, USB, or FireWire, can perform a variety of duties involving audio, MIDI, and other digitally controlled signals (such as synchronization). Also, many devices can perform all these tasks, which could make your interface a one-stop shopping solution for all your signal-routing and processing needs.
Analog Audio Interfaces
Analog audio is what comes out of your mic or instrument cable. It's what you would plug into a P. A. system or an amp, if you were playing live. But now we're in the digital recording world, so this signal needs to be converted to digital data to be recorded onto a computer. Inside an analog audio interface are analog-to-digital converters, chips and circuitry that turn continuously fluctuating voltages into discrete digital chunks. When audio is converted into digital data, two specifications are considered: the sample rate and the bit depth. The sample rate is how many times the converter samples (takes digital snapshots of) the incoming signal. Just as film projection involves moving a series of still photos past the eye fast enough to simulate continuous motion, so too is digital audio playback a series of frozen sonic images that, played sequentially at a fast enough rate, appears continuous.
Several sample rates exist, but the CD spec is 44.1 kHz (kilohertz), or 44,100 samples per second. Once these samples are captured, the next spec that comes into play is how to store them. A single sample at 16-bit resolution (16 bit means 2 to the 16th power) has a range of 65,536 different places to represent that signal. Higher sample rates and bit depths exist, and musicians are pushing to get these better formats as part of our available listening choices, but for now the CD spec of 44.1 kHz/16 bit is what all music must eventually become (even if it was originally recorded at higher and better sample rates and bit depths) if it is to be listened to on a conventional CD.