What's the difference between phonetics and phonology?

I'm surprised that nobody has mentioned the form/function distinction.

Phonetics studies the nature (acoustic and articulatory) of sounds that humans produce while speaking. Phonology studies their function in differentiating meaning in various contexts.

Thus (very crudely), phonetics will notice the difference between the aspirated and non-aspirated /p/ in pin vs. spin. But phonology will be mostly interested in the fact that the meaning of both words will change when you replace /p/ with /k/, in exactly the same way as in lip vs. lick where aspiration doesn't enter the picture.

But of course, phonology can only be successful when it has good phonetics to rely on.