A graduated neutral density filter is a filter that is half dark and half clear. It can normally be attached to the front of the lens with a lens holder, so that the position of the filter can be changed relative to the lens. Graduated ND filters are used to eliminate contrasts that are too high for the camera to capture, like a bright sky with a dark scene.
Although it is easy these days to change the brightness of part of an image in post-production, a graduated ND filter still comes in handy in overcast conditions. It brings out the foreground and can rescue an image from certain disaster.
How you can take a similar image
Lens: Wide angle lens.
Settings: Small aperture. Low ISO.
How: Use a tripod. Attach the ND filter so that the dark part of the filter falls over the bright overcast sky. Remember that the smaller the aperture, the more defined the line between light and dark will become.
Additional handy accessories include: reflector, flash soft box and flash tele-extenders, car-door bracket, bean bag, and battery packs for the camera or the flash. Make as many friends as possible.