Polarising filters work by suppressing reflections that occur on most surfaces. In the right conditions reflections on the surfaces of water and glass will vanish, allowing you to see more clearly through them.
But the contrast and colour saturation in your photos dramatically improves too. Blue skies become bluer, clouds stand out more, trees and grass look greener, and all the other colours in your scene seem richer. The transformation happens instantly, before your eyes, and cannot be replicated on a PC.
Polarising filters work best on sunny days with the sun at an angle of about 90° to the subject. On grey, overcast days they may have little or no effect.
ND Grad filters are clear in the bottom half of the filter with a gradual transition to the ND effect in the top half. They are primarily used for light reduction in skylines of landscape shots.