There are two kinds of projection we can apply to the graphics pipeline, Perspective and Orthographic. Perspective projection, like the name implies, views the world in perspective, there is a vanishing point somewhere in the distance. Orthographic projection, on the other hand, has no vanishing point. If two lines are parallel in an orthographic projection, they will never touch. For this reason, perspective projection is generally used to render 3D elements, and orthographic projection is generally used to render 2D elements:
When designing a projection matrix the most important thing is not the perspective, but the coordinate system. Depending on how we construct this projection matrix, the world will either be in a Left Handed Coordinate System or a Right Handed Coordinate System. The difference between these coordinate systems is the direction of the Z-Axis. In a Left Handed Coordinate System, +Z goes into the screen. In a Right Handed Coordinate System, +Z comes out...