By it’s colour? 😀
Only joking , Fanie. The question sound very basic and most people will think they know the answer. Yet the operative word in your question is not “what is”, but how do you “recognize” the difference, I presume in PP.
I typed a long explanation, but then remembered my old friend, Sean McHugh at Cambridge in Colour, so go over to http://www.cambridgeincolour.com/tutorials/noise2.htm and let him explain, as I fear my explanation will suffer in the Afrikaans to English back to Afrikaans translation. His knowledge on the subject is in any case orders of magnitude more than mine.