Last week, I mentioned that a rotating sphere will bulge out around its equator. Why? The answer has to do with centripetal acceleration--the linear momentum that tries to keep matter moving in a straight line, even when gravity has other ideas.
In this case, centripetal acceleration wants to fling matter off the planet, and the strength of the acceleration is related to how fast that matter is moving. That velocity maxes out at the equator.
So gravity and momentum come to a mathematical compromise: the Earth is a little fatter where it has its greatest linear velocity, at the equator.
Want to see the math? Of course you do! The ever-prolific Eric Weisstein has encyclopedized it here.