All 3-dimensional graphics data are expressed in terms of a
Cartesion **coordinate system**.

The coordinate system is a frame of reference - a location (origin) and 3 basic directions (coordinate axes). Everything is measured relative to this location and directions.

The OpenGL coordinate system is **right-handed** -
+X to the right, +Y up, +Z out of the screen.

In a left-handed coordinate system, +Z would point into the screen.

The specific directions are arbitrary, and often differ in different software packages or application domains.