I too have this camera and its an amazing piece of kit.
P mode (programmable auto) is like auto mode but it gives you some extra control over the exposure and various other things, for example if you use the dial you can choose from a number of shutter speed and aperture combinations (depending on how much light there is).
A (aperture priority) and S (shutter priority) will let you choose one on their own and the camera will try and compensate the other to get a good exposure.
M (manual) is full manual mode and the camera will not change any exposure settings automatically, although I think you can still choose if you want to turn auto ISO on or off in this mode. Personally I think auto ISO is a good thing for most photography situations and it does a good job, just keep the max ISO to 1600 or below as noise will start to become visible in the photos at higher ISO's.
Take a look on youtube as there as lots of good tutorial videos on how and when to use these modes.
Edited by Alex1M6 (Fri 26-Apr-13 18:19:05)