When you come right down to it, an audio interface is anything that lets you get audio signals in and out of your computer. The challenge in picking one is that there are so many ways of getting it done, and it will all depend on how you like to work and what you're trying to do. A very basic interface would be simply a box that converted analog audio to digital audio that the recording software in your computer can understand, and convert it back to analog audio when you want to hear it. Such converters are referred to as AD/DA (analog to digital/digital to analog) converters. Better quality converters have what's called "jitter correction" to compensate for small timing variations.
Most interfaces are far more than just converter boxes. They include mic preamps, multiple I/O (Input/Output) connections—balanced, unbalanced and digital—and even internal DSP (Digital Signal Processing) for effects, EQ and basic monitor or sub-mixing user. Some units can even act as a simple, stand-alone mixer without a computer attached for use as a basic live sound mixer.
One of the biggest determinants of overall audio quality in an audio interface is the quality of opamps (operational amplifiers) used in construction. Better systems will use lower-noise, lower-distortion components, or in some cases, discrete components rather than single-chip solutions. The number of I/O connections can be a deciding factor. The basic interfaces will be 2-in/2-out, able to handle a vocal and instrument together or a stereo pair of mics for capturing a live event. From there, you can expand as far as you need, depending on what you want to invest in your system. Interfaces with 18 inputs and 22 outputs are not uncommon and will handle most users needs, but you can go far larger with a full-blown Pro Tools HD rig, if that's what is required.
The final consideration is connectivity. There are multiple ways of connecting an audio interface to your computer—USB, FireWire, a PCI slot for desktop systems or Apple's Thunderbolt connection on newer Macintosh computers—all of which have different strengths and weaknesses. USB interfaces tend to be smaller, with most mobile interfaces using USB connections, but can be less robust when you need a lot of simultaneous ins and outs. FireWire, also called IEEE 1394, is fast and robust, but generally requires an external power supply. PCI cards are the fastest with the most bandwidth, but are tied to tower and desktop systems, so will be confined to a single studio location. Some of the more popular brands for interfaces are Akai, Apogee, Avid, Focusrite, M-Audio, MOTU, Presonus, Roland, Tascam and Yamaha, though there are many others. The best interface is the one you're going to use, so spend some time with the Pro Audio specialists at Guitar Center so they can help you find exactly the right one.
In addition to the mainstream audio interfaces described above, our GC Pro team of experts offers hands-on sales and support for professional solutions such as Avid HDX Systems and the Focusrite Rednet series. These solutions are typically used in recording studios, post facilities and other professional settings.