Refer to figure.

A radial is a magnetic bearing extending from a VOR station. Radials move along the compass rose in all directions. Radials follow the curvature of the earth, just like great circle tracks. As seen in the figure, great circle tracks run across the earth’s surface following the curvature of the earth. The rhumb line is used for planning a flight on a 2D map.

The Great Circle Track marks the shortest distance between 2 points, which is useful for navigation purposes. Due to convergency usually the track changes along the great circle. The equator and all the meridians (longitudes that run vertically across the earth). Any circle that is not a great circle is called a small circle, also known as parallel of latitudes.