2 things to remember: A stands for Analog while D stands for Digital. So if you see the term A/D, that stands for Analog-to-Digital converter while D/A is Digital-to-Analog. Usually, these converters are involved in ANY form of digital recording, whether is is to ADAT tape, DAT recorders, or computer recording.
A/D converters are responsible for the conversion of analog impulses, usually electrical, into digital ones and zeroes. Usually, the analog waveform is reconstructed in digital form so that it recreates the complex analog waves into digital representations that a computer or digital recorder can manipulate.
D/A converters, OTOH, reverse the process whereby the digital waveforms are converted back to analog so that you can hear it. These converters exist in anything from soundcards to mp3 players and cd walkmans.
How are these important to a computer-based musician or recording system? Simply put, the best A/D and D/A converters tend to accurately capture and convey sound. Poor converters usually are tiring to hear in the long run and often do not convey an accurate sense of 'space' in a recording (sort of like a collapsing stereo image). Part of this is because of poor clocks and high jitter in the converter circuitry. A good example will be to compare the sound output of a low quality consumer soundcard with a mastering grade soundcard. There is often a sense of airiness with very good cards that is pleasing to the ear and you can hear every nuance of a musical performance. Poor soundcards, aside from being noisy, often have poor imaging and sometimes do not accurately reproduce the audio frequency spectrum accurately.
