Projection matrix places all visible objects in a perfect view volume cube (shown on the right)
with coordinates between -1 and 1 on all axes:
This cube defines our view volume.
Projection transformation yields (x, y, z, w) coordinates such that
-1.0 <= x/w <= +1.0
-1.0 <= y/w <= +1.0
-1.0 <= z/w <= +1.0
These coordinates are called homogeneous clip coordinates, which define the clip space.
For more info, read http://www.opengl-tutorial.org/beginners-tutorials/tutorial-3-matrices/ matrices tutorial, which details how the projection matrix works.