CC,
CPL - Circular Polarizer: also referred to as polarizing filters, can be used to create circularly polarized light or alternatively to selectively absorb or pass clockwise and counter-clockwise circularly polarized light. (Other words make a sky more blue, for example).
I used the CPL on my Grand Canyon shots.
GND - Graduated Neutral Density: is a neutral density filter with different attenuation at different points, typically clear in one half shading into a higher density in the other. It can be used, for example, to photograph a scene with part in deep shadow and part brightly lit, where otherwise either the shadows would have no detail or the highlights burnt out.
ND - Neutral Density: is a filter of uniform density which attenuates light of all colors equally. It is used to allow a longer exposure (to create blur) or larger aperture (for selective focus) than required for correct exposure in the prevailing light conditions, without changing the tonal balance of the photograph.
Just thought be easier to copy from Wiki. =) Explains it's a bit better than I can and I hope this helps out!