Analog-to-Digital Converter, or ADC for short, is a device necessary to turn analog signals into digital so it can be read by computers or other devices that require such input.
All ADC convert analog signals into digital binary formats.
The most common use for ADC are for converting old movies or music tapes to newer digital formats. Tapes like VHS or cassette can then be converted to AVI or MP3, for example.
On the opposite end, occasionally computers have to output analog signals and a digital-to-analog converter (DAC) is required for such action.
Common uses include outputting digital content to an analog television or digital audio to a CD player.