What is the difference between an ADC and a digital input pin?
The difference is that a digital pin is only able to detect whether there is
a voltage present on a pin or not, while an analog pin is connected to an
internal Analog to Digital Converter (ADC) and can measure
the actual voltage on the pin.
A digital pin is suited to detect if a button has been pressed, while an
analog pin might for example measure the position of a potentiometer.