"0" or "A0" when used with analogRead()?

When you use the analogRead() function, "0" and "A0" refer to the same pin, the analog pin 0.

These two invocations of analogRead() are equivalent:

analogRead(0);

analogRead(A0);

Therefore, when you use analogRead() or analogWrite(), it doesn't matter which pin name or number you use.

However, there's a "catch".

It is possible to use an Arduino analog pin as if it was a digital pin.

For example, you can write:

digitalRead(A0);

This will return either HIGH or LOW, instead of a number from 0 to 1023 (assuming you are using an Arduino Uno).

Points to remember:

• If you use an analog pin 0 with digitalWrite() or digitalRead(), then you must use the full analog pin designator ("A0", "A1", etc). 
• If you use an analog pin with analogRead() or analogWrite(), you can simply use the number of the analog pin ("0", "1", etc.) or the full analog pin name ("A0", "A1", etc.).