Perspective  the simplified basics
If you stand in the middle of a long straight street and look down it, the sides of the street appear to get closer together as they get farther away. This phenomenon is a consequence of our visual apparatus  a creature with 100 eyes spread over a square mile would percieve the street in a different way.
We use "perspective drawing" to represent on paper the way we perceive 3dimensional objects in space. It's possible to construct a geometric model that includes an observer, and plot the way that observer will perceive an object in 3  space. This geometry is called "single plane" or "perspective" projection. One can do this using mechanical drafting, by hand or computer, but it's important to be able to sketch freehand perspective as well.
The fundamental rules of freehand perspective drawing are relatively simple
  objects, lines, or surfaces appear to diminish in size as they become more distant from the observer. This means that....
  parallel lines or objects appear to converge as they become more distant from the observer
 AND
  the point at which such lines would seem to meet if they were infinitely extended is called a "vanishing point"
  in a front view (the one we most commonly see), vanishing points are located on the "horizon" which is normally perceived by the observer to be at her eye level (even when not actually visible).
