http://www.thedigitalcamerashop.co.uk/prod...ils.php?id=2513
The filters are a neutral grey, darker at one side, fading to clear. The idea is to use them when the sky is much brighter than the land. You've no doubt noticed that the land is often underexposed, compared to the sky. Your camera is really good at the sky bit, so you're unlikely to get blown highlights in the clouds, but it can make the land look horribly dark.
The best way to get the filters is in a set, but don't get the "landscape" set, get the ND grad set. They are quite expensive though, but I will eventually be getti