3D Geometry
Fundamental Principles
Cartesian Coordinate System
Cartesian Coordinates were invented by a French mathematician René Descartes (or Renatus Cartesius in Latin, hence the name 'Cartesian'). The idea was that everything has an origin from which 3 axis emerge, and the placement of an object/point is signified by the distance and direction of that origin.
Cartesian coordinates allow one to specify the location of a point in the plane, or in three-dimensional space. The Cartesian coordinates (also called rectangular coordinates) of a point are a pair of numbers (in two-dimensions) or a triplet of numbers (in three-dimensions) that specified signed distances from the coordinate axis. http://mathinsight.org/cartesian_coordinates
The system is continued to be used in many programmes, xy axis in 2D software like Adobe Illustrator or Photoshop, xyz axis in 3D software for making 3D models. While making a 3D model out of polygons, the person really signifies that positioning on plane or in space of points, joins them in edged which then create polygons and finally a mesh.
In both of these, the image representation is suggested by the record of the points' co-ordinates in the working area.
Primitives
Primitives are usually (but not restricted to) the most basic 3D shapes, such as spheres, cubes, cones, pyramids, cylinders etc. They are already put into a software and can be created manually as well. Their role is to be a starting point in polygonal 3D modelling, and can be easily modified once created.
Surfaces
Other surfaces are normal and bump maps and allow further distortion of a model without adding to the polygon count.