DOF is the distance between the nearest and farthest objects in a photograph that appear reasonably sharp. Think of this area as the focal plane as well. Lenses can precisely focus on a given point. However, anything closer or further away from that point begins to leave the focal plane. This is how you get the nice blurry backgrounds you see in some photographs or the tack sharpness in others.
Have you ever heard of using the back button to focus on your DSLR? It seemed very odd and foreign to me when I first heard of this. With my finger already depressing the shutter ½ way down to focus it was easy to actuate the shutter. The thought of adding an extra step in how I shoot make very little sense to me. Still, I kept hearing about this back button focus thing. So I decided to give it a try. Here is what I discovered