@christiq an ND filter holds back some light, this is useful if you want to shoot wide open in broad daylight and still keep a low shutter speed. A GND (gradual ND) filter, only blocks light in the top half, useful when there is too much contrast between the sky and the foreground. A polarizer adds contrast and reduces reflections. All these effects can also be accomplished in photoshop, but it is way more effective and natural if you use filters for it.
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I used a .9 Graduated ND Filter to capture this shot. Otherwise, my sky would have been completely blown out, or the ground a silhouette.
I've also used .9 ND Filters to capture ocean waves in daylight, but I haven't used any of those photos on here so I can't link to them.
Used a polarizer not an ND filter. And adjusted some contrast in PS. That's it though.