[UPDATE (April 5, 2011): It turns out some of the descriptions I used below to describe a geoid were not accurate. I refer you to this page at the University of Oklahoma for a good description. I've made some changes below to hopefully ease any confusion.]
Most people think of the Earth as being a sphere. For most purposes that's close enough, but it's actually a spheroid, something close to but not precisely a perfect sphere. It bulges in the middle (as so many of us do) due to its spin, the Moon's gravity warps it, the continents and oceans distort the shape. And the surface gravity changes with all this too; it's different on top of the highest mountain, for example, compared to its strength in Death Valley. So if you could map out the average shape of the Earth's gravity, a shape where the gravity is the same no matter where you stood on it, what would it look like? So if you could map out the Earth's gravity -- essentially, a diagram showing you the direction of "down" -- what would it look like? It would look like this:
