Understanding Exposure Mode

There are five major settings on your camera that help you to change your shutter speed and aperture. 

 P is short for programmed autoexposure, and this is when the camera automatically chooses the aperture and shutter speed, but you can choose from different combinations of the two.  

Tv stands for shutter-priority autoexposure, and this is when you pick a shutter speed, but the camera automatically chooses an appropriate aperture setting to match the shutter speed.  

Av means aperture-priority autoexposure, you can pick the aperture, and the camera automatically picks a good shutter speed to match.

A-Dep, auto depth of field, is where the camera evaluates the distance between your lens and the objects in your photo,  and then chooses the best aperture to keep the objects in focus.

Finally, M, is manual.  In manual you can choose both the shutter speed and the aperture, but for best results you should use the light meter, and then try to get the light meter to zero.