The shape of the Earth is classified as an ellipsoid or an oblate spheroid. The Earth is not considered a true sphere because it has a larger circumference and diameter at the equator. The poles of the Earth are squished, which creates a slight bulge around the equator.
Gravity is a force that causes planets to form the shape of a sphere by pulling the mass of the planet close to the center of gravity. In the case of the Earth, gravity pulls the planet as close to the core as possible. The sphere-like shape of the Earth is distorted slightly by the centrifugal force of its rotation. This force causes the mass of the planet to move away from the center of gravity, which is located at the equator. For that reason, there is a minor outward bulge at the equator, creating a larger circumference and diameter.