|DMC-LX5, f2.3, Shutter 1/2000|
|DMC-LX5, f5.0, Shutter 1/1000|
There's nothing wrong with owning both - an enthusiast compact for your pocket or purse, with you at all times comes highly recommended, and either a DSLR or MILC for around your neck when you know you're going to do some serious shooting.
So, back to PSAM... every DSLR and MILC has these letters on the top dial, or in the case of certain super-pro models, some other way of getting at this functionality. These letters stand for the following:
"P" = Program - a Mode in which the camera sets most of the values for you, but certain things are still "Programmable" by the photographer, such as picture size, RAW or not RAW, ISO Auto or Manual, exposure compensation (brighter or darker), white balance and flash function. Some models also include "Program Shift", which behaves more like "M" Mode (see below)
"S" - Shutter Speed Priority, also known as "Tv" for "Time Value" on Canon cameras. This is an automatic mode that allows the user to choose the shutter speed, and optionally all the other things available in "P", but the camera chooses the Aperture to give correct exposure. One example of this would be to capture the detail of a full moon, you would need to choose a much faster shutter speed than would normally be used at night.
"A" - Aperture Priority Mode, also known as "Av" - Aperture Value on Canon cameras. This is an automatic mode that allows the user to choose the lens opening or aperture size, and optionally all the other things available in "P", but the camera chooses the shutter speed to give correct exposure. This is the most used mode. Aperture size is what determines what parts of a photo are in sharp focus, and also has the most impact on the biggest range of variables in picture quality. Consider the two shots above - there's a subtle but noticeable improvement in quality of the second picture, because I chose a one-stop smaller (numerically higher) aperture setting.
"M" - Manual Mode. This is not an automatic mode, although your auto-focus will still work. Everything in "M" Mode related to exposure needs to be set by the photographer, including both the shutter speed and aperture value. As I mentioned above, for "P" Mode where some cameras (not all) permit a Program Shift, this is primarily what is accomplished in "M". If you select a slower shutter to let in more light, you need to compensate with a smaller (numerically higher) aperture to let in less light, and vice-versa. This is also known as a "Program Shift". This is easily accomplished on cameras that have dual control wheels - one for shutter and one for aperture, or with a lens which has an aperture ring. It's rather useless, however, if you need to hold a button while turning a wheel to select one or the other values. "M" Mode is particularly valuable if you're using older, high quality manual lenses where you're selecting the aperture value and focussing manually via the lens rings anyway.
My advice is to stick with Aperture Priority shooting, as it will provide the ultimate control over picture quality. Occasionally, you might prefer to use "P" Mode, especially when using flash, because flash is a big part of the camera's automation, and in some situations does not come off quite right in the other modes, unless you really know what you're doing with flash.