Graduated ND filter should become a natural part of your photobag. Even in this digital world
Shortly explained: The GND filter reduce the amount of light in part of the motive, ie. the skies, and help you balance out the different amount of light.
8x reduce the amount of light with 3 stop, 4x= 2 stop and 2x = 1 stop. Remember that this appear in just on side of the filter as it graduate from non effect to full effect with different "hardness".
A polarizing filter remove the reflects and increase /enhance the color. It also reduce the amount of light hitting the sensor and force you to take this into consideration when exposing.
This filter have most effect when the light appear from the side, or more behind you. With a sircular Pola.filter you're able to change the effect by turning the filter.
Hope this help you.