Analog to Digital Converter: An integrated circuit (IC) which converts an analog signal to a digital representation. As an example let's consider an 8-bit ADC with a 5.00V reference.

8-bit:
An "8-bit ADC" means that it will convert the input signal to an 2^8 equivalent numerical representation (that's where the "8" comes in.) 2^8 = 2*2*2*2*2*2*2*2 = 256 possible values. In the digital world, zero is a legitimate number, which uses up one of the possible combinations, giving us a 0-255 count output.

5.00V reference:
The reference voltage is what the input signal is compared to, as a fraction. If we put a 5.00V signal into our ADC, and it's using a 5.00V reference, what will be the output? Full scale! In our 8-bit example "full scale" is 255 counts so that's what we get for the output. A zero signal input will output zero. Anyway the math works out to:

Digital output = ((Vin / Vref) * 2^number of bits) - 1

With this you can find out exactly what your software will read from the ADC with a given input signal.

(For the purpose of this example I left out the case of measuring both positive and negative voltages, we won't see that in an automotive ECU.)

Analog to digital conversion is *never* perfect. It is a process of approximation - each *exact* digital value has to represent a *range* of analog values. The resolution (normally measured in bits) of an analog to digital converter represents how many possible values the analog range can be divided into.