2.8 Geographic and projected coordinate reference systems (CRS)
2.8.1 Geographic coordinate systems
Longitude is location in the East-West direction in angular distance from the Prime Meridian plane. Latitude is angular distance North or South of the equatorial plane.
Earth is not flat! and not exactly a sphere (neither an ellipsoids)
To define an ellipsoids we need the polar (semi-minor axis) and equatorial radius (semi-major axis).
The datum (geocentruc / local) contains information on what kind of ellispoid use and the relation with Cartesian coordinates on earth surface.