P, S, A, and M modes are referred to as "exposure modes" because they let you control settings that determine exposure, namely shutter speed and aperture.

Each of these modes offer different degrees of control over shutter speed and aperture.

Mode P (Programmed Auto)
Recommended for snapshots and in other situations in which there is little time to adjust camera settings. Camera sets shutter speed and aperture for optimal exposure.

Mode S (Shutter-Priority Auto)
Use to freeze or blur motion. User chooses shutter speed; camera selects aperture for best results.

Mode A (Aperture-Priority Auto)
Use to blur background or bring both foreground and background into focus. User chooses aperture; camera selects shutter speed for best results.

Mode M (Manual)
User controls both shutter speed and aperture. Set shutter speed to “Bulb” or “Time” for long time-exposures.