An oblate spheroid is a special case of an ellipsoid where two of the axes are of the same measurement (see diagram).  An oblate spheroid is created when an ellipse is rotated around one of its axes.  This is seen in nature where objects are rotating around an axis like the Earth rotating round the polar axis.  For this reason, the Earth is often modeled as an oblate spheroid.  This helps account for the equatorial bulge of the Earth created by centrifugal forces that counteract the inward force of gravity.