What is Latitude and longitude?

Let the Earth be a sphere turning about its polar axis. The latitude $\varphi$ of a point is the angle between the equatorial plane and the radius through the point, from $-90{}^{\circ}$ (South Pole) to $+90{}^{\circ}$ (North Pole); its longitude $\lambda$ is the angle, measured in the equatorial plane, between the meridian half-plane of the point and that of a reference meridian, the one through Greenwich, counted positive to the east, from $-180{}^{\circ}$ to $180{}^{\circ}$.