Professional audio production is one of the most digitally-oriented creative careers in the world. These days, digital audio workstations and computer software have largely displaced the hardware mixers, synths and sequencers of the past. We still rely on analog signals from the microphone in recording and to the speakers during playback, however, which means that audio converters are an important part of any studio. These work like translators, changing analog signals to digital ones (or vice-versa) to allow analog and digital components to communicate with each other.
Any all-in-one recording device will have its own built-in analog-to-digital converters, and any hardware that plays digital recordings has its own digital-to-analog converters. The catch with these integrated converters is that they're a compromise, made as compact as possible so that they'll fit into the equipment. For general-purpose recording and playback, that's fine—but for professional studio use, where you need the absolute highest sound quality, there's no substitute for a dedicated converter.
Depending on your studio setup, you'll either be looking at a desktop converter or a rack-mounted one. Most of the models can convert one way or the other, but some are dedicated DACs or ADCs, so take note that you're choosing the right one for your needs. A few examples of desktop converters are the Antelope Audio Integrity Zodiac, Lynx Hilo or Benchmark DAC1. These are all good choices for mobile recording studios that need to pack their equipment up from time to time for transport. Most of the others are rack-mounted, which makes them especially useful as interconnects between digital and analog equipment they may share a rack with in your studio. If you're a professional power user with no holds barred, the Antelope Audio ECLIPSE 384, with its atomic clock, might be the right choice for you. If you're a hobbyist looking to upgrade your home studio, you'll probably want a more down-to-earth convertor.
Maybe you're using primarily software editing tools, in which case, one detail to pay particular attention to is the selection of connectors on a given converter. Mac users will appreciate the Lynx models with Thunderbolt support, or if you're using a PC, look for USB or firewire. All of the audio converters available here are good choices—it's just a matter of matching the right model to your own studio setup. Will it be going into a rack or sitting on a shelf? Take your time and compare the features of the different models, and you'll find the one that's best for you soon enough.