Free body diagrams (otherwise known as FBD's) are simplified representations in a problem of an object (the body), and the force vectors acting on it. This body is free because the diagram will show it without its surroundings; i.e. the body is 'free' of its environment. This eliminates unnecessary information which might be given in a problem.
Let's take this figure to be a pictorial representation of our problem: a sled on snow, with horses pulling it. First we will represent the sled (the 'body' in our problem) as a (really) simplified figure, a square resting on a flat surface.
In this tutorial, we will review some of the main forces which you will encounter in physics, and discuss their contribution to an FBD.