Digital cameras can have dozens of different shooting modes, from the standard "Automatic" mode that lets the camera handle everything but the shutter and zoom, to more specific modes like "Fireworks" and "Birthday Party." The majority of these mods are straightforward presets, locking the camera into certain exposure settings commonly used in those situations. While they might seem helpful on point-and-shoot cameras, they offer no real control over how to shoot.
Program, or "P," is essentially the automatic shooting mode for skilled photographers. In Program mode, the camera handles the aperture and shutter settings, but lets you dictate ISO sensitivity and, when the camera supports it, lets you shoot RAW image files. Those choices are an important distinction from most cameras' Automatic modes, which automate the ISO sensitivity and usually only let you shoot JPG images, even if the camera supports the RAW image format.
Shutter Priority, or "S" (sometimes "Tv" on mode dials), gives the users full control over shutter speed and adjusts the lens aperture to get a proper exposure. If the camera doesn't think you can get a good exposure at your chosen shutter speed based on the light metering, the display will usually show the f-stop in red, indicating it's not enough. This mode is best for setting up high-speed action shots or long exposures for artistic effects. If you want to capture a thrown ball or a playing dog, crank the shutter speed to 1/250ths of a second or faster. If you want a very long exposure of the moon, or an artsy photo of cars whizzing by a dark street at night, set the camera on a tripod and bring the shutter speed down to half a second or slower.
Aperture Priority, or "A" is the opposite of Shutter Priority; this mode mode gives the user total control over the lens' aperture, automatically setting the shutter speed to provide an appropriate exposure at the chosen f-stop. This mode is useful for controlling the depth of your focal plane in a shot. By opening the aperture wide, to f/1.8, you can get a very narrow focal plane that's perfect for artistic shots and portraits.
This technique lets you capture a very sharp subject while leaving the background extremely blurry, causing the subject to "pop." Alternately, you can get a much wider depth of field by closing the aperture down to f/8 or higher, which will put in focus much more in front and behind your subject. While Aperture Priority mode doesn't have an indicator that shows if an exposure will be poor, it's best to stick to 1/50th of a second or faster shutter speed when shooting without a tripod. If the camera cranks the shutter speed slower than that using your aperture setting, consider other shooting options.
Finally, Manual, or "M," is the total user control mode. It lets the user change both aperture and shutter settings manually, giving both the most flexibility and the most potential for a bad exposure. Because of this, you should only start using Manual mode after you've gotten used to the others and see what different settings mean in different conditions. Fortunately, like the Shutter Priority mode, the Manual mode on many cameras have an indicator that turns red if the current settings will produce a bad exposure under current lighting. No automatic indicator can make up for experience, though.
What shooting mode do you favor? What are your tips for shooting with Program, Shutter, Aperture, or Manual modes?