Orthographic Projection consists of drawing 2d drawings of 3d objects, that is, plans, elevations, end views and auxiliary views.
History of Orthographic Projection:
There are two main systems used in Orthographic Projection, those been First and Third Angle Projection.
First Angle Projection:
Shown below is an interactive 3d Model of an object in Orthographic Projection.
Rotate the 3d object around and see how the Plan and Elevation are obtained by looking straight in at the Top or Front of the 3d model.