The viewfinder of your SLR tells you almost everything you need to know about the shot you are about to take. One thing that it doesn't show you, however, is the amount of light that comes through at the aperture you specify. If you're one of the lucky few that have a depth of field preview button, you can actually see the difference in light when you change apertures by depressing it. What you'll also be able to see is the change in depth of field. And, now that I've been throwing that term around for a paragraph, I'll get to explaining what it is.
First, we need a bried explanation of focus. When you (or the camera) turn the focusing ring of the lens to focus on an object, the focal length of the lens (f') is changed to accomodate the following (simplified) formula: 1/f' = 1/Do + 1/Di. Di is the distance from the lens to the film surface, which is always fixed. Do is the distance from the lens to the object on which you want to focus. So, when a lens is set to a focal length of f' and the plane of the film is Di away from the lens, objects that lie in the plane that is Do away from the lens will show up in focus. Things that lie outside of that perpendicular plane aren't necessarily in focus.
Secondly, we need to understand a little more about the shapes of the glass elements of the lens. The surfaces of these elements cannot be flat because they most focus light on a particular distance. However, lens concavity and convexity (measures of the curvature of the surface of the lens) distort the image by bending the rays of light that they transmit.
And, this is where I started school... so no more time to write... :(
