An Analogue Comparator allows you to trip your input pin at different voltage levels determined by an internal or external reference. So, if you set your Reference voltage at say 500mV, you can trip the Comparators Output on or off at 500mV rather than the usual trip level for a Digital I/O pin.

By sweeping the Comparators Reference Voltage, you can then use its input as a cheapo ADC. However, the internal Reference in the 16F628 only has two scales of 16 steps, and allowing for a settling time would not be very good for Audio input sampling.

So, in reply to your query, an Analogue COMPARATOR does what it says... it COMPARES your input against a Reference. Whilst an Analogue to Digital CONVERTOR (ADC) will give you a snap-shot of your current Analogue input in Digital form. The two are NOT the same.

The 16F628 has TWO COMPARATORS on-board and NO ADC's. ADCIN only functions with ADC's which is why it's named ADCIN (that what I love about microprocessors, they name everything so obviously!).