Now just to complicate things, ND Grads are available in Hard and Soft gradations. I use the Lee Filters system and I find that despite the name, Hard Edge Grads are just perfect for landscape photography and don't give you a hard line or edge running through the image. Ninety-nine times out of 100 I will use a Hard Edge Grad for my landscape images. Occasionally though, a Soft Edge Grad can be indispensable. When working in woodland where the trees are top-lit and you just want to hold the exposure back in the upper portion of the image, a Hard Edge Grad will produce too much of an exposure difference and you will end up with tree trunks going from light to dark. With a Soft Edge Grad the gradation is very subtle and less pronounced, which is ideal when you have subject matter running into the graduated portion of the picture.
One important point is to set the camera to manual exposure before shooting. If you have the camera set on a Program mode, the meter will think the sky has gone dark when you apply the ND Grad and will try to compensate and make the image brighter, thus negating the use of the grad and leaving you back to square one.
The more you stop down a lens, the more a graduation becomes apparent. With digital cameras, unless you need a huge depth of field, you should normally be working at about f/8 to f/11 as these apertures will be using the sweet spot for sharpest results from the lens. These apertures are ideal for Hard Edge grads as they will show no line or edge through your image at all, but you do have to be wary of your focal length. The wider the focal length of your lens, the sharper a Hard Edge Grad may be, so you may wish to switch to a Soft Edge Grad to counter this. Certainly once you are working with extremely wide lenses a Soft Edge Grad will be a must.
Once you start to play with Neutral Density filters and Grads you'll realise how simple it is, and you'll soon be able to start combining the two. Use the ND Grad to control the exposure difference between the foreground and sky, and apply the overall ND to increase the exposure of the whole image, as to create subject blur or movement.