Free Body Diagrams

This free body diagram a shows an object. This object could be a chair, a box, or some other thing. I'll say a box.

The forces shown on the diagram are Gravity, Friction, the normal force, and Fapp

Gravity is in the object Because objects always have gravity. (We will assume the box is on earth...)

The normal force is there because the box is actually on a table. The normal force is shown when an object is on a surface because it can't fall or accelerate down.

The Fapp force is there because there is a constant force pushing it to the right. 

The friction is there too, and since it is only "half" of the Fapp, the box is moving.