Take a moment to look at any photograph, painting, picture in a magazine, or look out your window.  No matter where you look, or what you are looking at, you can separate the visual components into "Foreground" (the parts of the scene closest to you), "Background" (the parts that are furthest away) and "Middle Ground" (everything inbetween the closest and the furthest).

When photographers talk about Depth of Field, they are talking about how many of these components are in sharp focus.

If everything in an image is in sharp focus, photographers say there is "extensive depth of field" or "great depth of field" or even "lots of depth of field".  Think of landscape shots.  Think of your own property listing shots.

If only the foreground, of parts of the middle ground are in sharp focus, photographers say there is "shallow depth of field", or "limited depth of field".  Why would you want parts of your photo out-of-focus?  To draw attention to the main subject.  Think of a portrait.  Think of arty shots of flowers.  Think food photos in gourmet cooking magazines.

So how does the photographer control Depth of Field to achieve either effect?  By changing the size of the circular opening in the lens that allows light to strike the film or sensor.  This opening is called the Aperture.  And by changing the shutter speed to work in tandem with the aperture size.

Here is the fundamental concept in (I hope) very simple terms:

"X" amount of light is needed to create a photographic image.

That light reaches the camera's sensor (or film) in two different ways.

First way:  The size of the Aperture opening.

Second way:  The amount of time that the shutter stays open.

By varying those two ways in different combinations, the photographer achieves different effects.

For instance, smaller aperture opening plus slower shutter speed  equals great depth of field for landscape or architectural photos.

Larger aperture plus faster shutter equals shallow depth of field for portraits.

For more info about lenses:  See Bryce's excellent post here...

Next up:  Understanding ISO