Why does this cat let me pet him and then nip?

My neighbours have this cat, he's a male, I don't know what type he is, but he's gray and white patches with black stripes. Anyways, every time I pat him, he starts purring and rubbing his head against me. But then he (still purring) might nip at my...

fingers, or scratch me.

What does this mean?

he wants to play or he wants you to do something differant. . . what he is doing is actually his way of showing affection. Remember Cats play ruff. . . . using both claws and teeth. . .

yes he is showing you love. thats why they say love bites lol

The rubbing in not just showing affection it is putting his scent on you. He is marking you as his own. He loves you and is treating you with affection in his mind.

Cats can suffer from overstimulation. One minute you can be petting him and he'll be purring, then the next he'll have your hand in his mouth. It's just his way of saying "I've had enough.". All you have to do is stop petting him if he looks like he's getting agitated.