The shape of the Earth is best described as an oblate spheroid. This means that the Earth is approximately spherical but slightly flattened at the poles and bulging at the equator. The equatorial diameter of the Earth is slightly larger than the polar diameter, resulting in an ellipsoidal shape. This deviation from a perfect sphere is primarily due to the Earth's rotation, which causes the equatorial region to experience centrifugal force, pushing material away from the axis of rotation and causing the bulge at the equator.
However, it's important to note that the Earth is not a perfect oblate spheroid either. The Earth's surface is uneven and exhibits various irregularities such as mountains, valleys, and oceanic trenches. These features, along with other factors like gravitational anomalies and variations in density, cause the Earth's shape to deviate slightly from a perfect oblate spheroid. To represent these irregularities, scientists use a more precise model called the geoid, which is a surface of equal gravitational potential that closely approximates the Earth's shape and accounts for these variations.