Aperture Priority
The Aperture priority you will come to learn has everything to do with the amount of light is let into your camera. If there is too much light then you are probably over exposing your photo and if there is too little light then you are under exposing your photo. Aperture is usually found when you switch to your semi manual or manual mode. This priority controls the length in which you will open the shutter of your camera and will determine how much light comes into your camera with the help of the shutter priority. The shutter priority aids the aperture priority by controlling how fast your shutters will open and for how long they will stay open.The size of your camera also comes into play when the aperture priority is being set. This is because there are different kinds of cameras and each depending how big they are will have to be set right.
If you are trying to change the depth of field then you usually use the aperture setting and even though shutter priority is pretty much in the same league as aperture, it is often used by semi professional photographers, especially those that take photos for sports where they have to capture the players doing their thing in slow motion or before the play is done.