An analog-to-digital converter (ADC) is a device that can convert an analog signal into a digital one. The conversion involves quantization of the input, and instead of continuously performing the conversion, an ADC does the conversion periodically by sampling the input at specific moments. The result is a sequence of digital values (having a well-defined resolution, that is, the number of bits used to represent the converted digital value) that have been converted from a continuous time and continuous amplitude analog signal to a discrete time and discrete amplitude digital signal.
As a simple example, in the following graph, there is an 8-level ADC coding scheme where the input signal Vin is referred to with the Vref signal (the maximum allowed input value) and then encoded into a binary number:
