P, M, A, and S are, of course, the four main exposure control modes of the modern digital camera: Programmed/Auto (P), Manual (M), Aperture Priority (A), and Shutter Priority (S).
I have felt for some time that those purists who argue a “true” photographer should only use full manual exposure are mistaken. There are good times to use all four. Note that I specifically do not include full automatic or any of the scene modes that come on some cameras. But the four basic ones – P, M, A, and S – should provide a good repertoire of exposure control methods for the sophisticated photographer, and I recommend that one be knowledgeable about and comfortable using all four.
An excellent series of articles has begun at the Digital Photography School – Aperture Priority and Shutter Priority: Exposure Lesson #1. This lesson provides a great survey of the uses of each aperture and shutter priority modes, and some great exercises to practice these and see how they can be useful tools for the photographer in a variety of shooting situations. Highly recommended.