Geographic coordinate systems

A geographic coordinate system (GCS) is a system for identifying points or areas on the surface of the earth. (It's usually the earth, but it could also be the moon or Mars or some other spherical body.)

The latitude-longitude system
The main geographic coordinate system is the latitude-longitude system, which you already know about. Lines of latitude are parallel circles that run east–west around the globe, getting smaller as they approach the poles. Their values range from +90° (north pole) to -90° (south pole), with the equator being 0°. Lines of longitude are half-circles that run north–south from pole to pole. Their values range from 0° to 360°, with 0° being arbitrary, but commonly designated as the line that passes through Greenwich, England.

The network of intersecting lines is called a graticule.

Lines of latitude and longitude form a mesh called a graticule. Latitude lines (parallels) are shown in yellow. Longitude lines (meridians) are white.

Defining latitude and longitude
When the earth is treated as a sphere, latitude is the angle between two lines drawn to the center of the earth: one from a point on the equator and the other from a specified point on the same meridian.

Longitude is an angle between two lines drawn to the center of the earth from the equator: one at the prime meridian and the other at a specified meridian.

Latitude and longitude defined on a sphere.

When the earth is treated as a spheroid, latitude is the angle between two lines drawn toward the center of the earth: one from a point on the equator and the other normal (perpendicular) from a plane that touches the spheroid at a specified point.

Latitude defined on a spheroid. The lines don't intersect at the center of the earth (unless the plane is tangent at a pole).

Longitude is defined on a spheroid the same as it is on a sphere.