A Neutral Density Graduated filter (also known as an ND Grad) is very similar to a Neutral Density filter but has one major difference. Instead of covering the whole filter and therefore affecting the whole of the image and exposure, it will run from a clear filter and gently fade into a neutral density, allowing only part of the image to be affected.
ND Grads are used primarily to control the exposure between the foreground and sky of an image. Usually the sky will be the brightest part of the image and the foreground the darkest. Meter for the foreground and the sky will be massively overexposed; meter for the sky and the foreground will be too dark. The solution is to meter for the foreground (the darker area), meter for the sky (the brighter area) and note how much brighter the sky is - probably two, three or four stops. Set the camera to the exposure reading for the foreground and use an ND Grad of the appropriate strength (two, three or four stops) to darken the sky, thus balancing out the exposure of both foreground and sky. After a little practice, the procedure of metering for the foreground, metering for the sky and applying an ND Grad will become second nature.
How to use an ND Grad
Take a meter reading from the foreground. A shot taken at this setting would have an overexposed sky.
Take a meter reading from the sky. A shot taken at this setting would have an underexposed foreground.
Work out the difference in stops (eg. 3) Expose for the foreground, and add the appropriate Grad (eg 3 stop) to darken the sky.