Excellent explanation bb, in most cases you'll find N-maps used on game models, the original models are very hi-poly, but those are not to much use in a game, so the hi-poly models are detailed up in progs like ZBrush or 3DCoat, then the model retopologised down to lo-poly models, using programs like, again, 3DCoat or TopoGun, but these programs transfer the hi-poly UVmapping from the original model to the lo-poly one and it's just the bump & normal maps that make these models look very similar to the original hi-poly ones, but without the high CPU/GPU processing needed to run them in a game