Instead of the Earth being a perfect sphere, due to gravitational and rotational forces, a simplified shape of Earth more resembles an oblate spheroid.

Via Wikipedia

Lingering from the creation of the Earth from a massive spinning disk of gas and dust, a point at the Earth’s equator is still rotating at 1,525 feet per second (465m/s). These high-speed revolutions cause centrifugal forces to develop, encouraging material to be flung outwards. As a result, the Earth cannot be strictly described as a sphere, and is slightly compressed at both poles by about 13 miles (21 km), giving it this odd shape.