Each mode is based on how the camera sets the exposure.
Digital SLR cameras typically come with four exposure modes: automatic, shutter priority, aperture priority and manual. While none of these are necessarily better than another, each has a specific situation and user skill level for which it's most appropriate. By learning how each mode works, you'll be more versatile as a photographer when difficult situations arise.
Automatic Mode
Automatic mode is what most users are familiar with. In automatic mode, the camera will adjust all of the camera's settings by itself based on what the sensor detects. This is the easiest way to shoot a quick, evenly exposed photo. It may not create a quality exposure 100 percent of the time, but it will for most situations. This mode is best used for casual photos and for scenarios when you don't have the time or energy to worry about the exposure. Automatic mode is also sometimes called "program" mode.
Shutter Priority
Shutter priority, which is typically abbreviated as "Tv" or "S" on shooting dials, allows you to the set the shutter speed to be constant and the camera will adjust the other settings to produce a quality exposure. This mode is best used for action or high-movement photography when you want an even amount of motion blur throughout the photos. The disadvantage to shutter priority mode is that the brightness of the exposure is limited to the maximum width of your lens aperture.
Aperture Priority
Aperture priority, which is typically abbreviated as "Av" or "A" on shooting dials, allows you to the set the aperture width to be constant, and the camera will adjust the other settings to produce a quality exposure. Use this mode when you want the same depth of field, which is the amount of blurring in the foreground and background, between each photo. Typically, this works best in bright situations where the shutter speed can vary wildly without affecting the photo. The disadvantage to this mode is that the camera may make the shutter speed extremely slow, such as 1/10 second, if the available light isn't enough.
Manual
In manual mode, you adjust every setting of the exposure -- aperture width, shutter speed -- and the camera will shoot as you dictate. Typically, this is the hardest mode to shoot in because you must adjust the camera every time the light in your composition changes. However, some photographers prefer to use this mode most of the time so they have complete control over the image. Additionally, using manual mode often builds an instinct of how you'll need to set a camera given a certain situation.
