Tom,

A good idea without getting long winded:

Automatic Mode:This is point and shoot mode. The camera selects what "it thinks" is the best exposure based on available lighting.

Aperture Priority: The user selects the f stop or aperture, the camera automatically selects the shutter speed. Use when you need to control depth of field. Do not use when freezing the action is important. A tripod might be needed for long exposure times at higher numbered f stops.

Shutter Priority: The user selects the speed, the camera selects the aperture or f stop. Use when freezing the action is important like sports events. Don't use when focus at different distances is critical. For instance you might want to photograph a person but want the background in focus too.

Programmed Mode: The camera has predetermined combinations of both aperture and shutter. It's really just marrying the other two modes together. It all does the same thing.

My suggestion is when taking some photos take some in the automatic mode, then also take some using the semi-auto modes and see how the different setting affect the results. I rarely shoot in automatic mode especially if I have the time to compose my subject.

Best,
Warren