Exposure

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When the camera is put in "P" mode you get to control the ISO's and the exposure level. The exposure level is not a physical setting but more of a guideline for how your camera should think when given a shooting situation. In any mode (besides Manual Mode) the camera has to do some thinking and decision making. 

The exposure setting tells your camera how to think when it needs to make a decision. exposure is commonly refered to at (ev) and talked about in terms of f-stops. For example, ev+2 mean exposure is 2 f stop higher then a neutral exposure. 

Lets look at some picture examples.

If you see the below picture of a statue it is at ev=0. Basically my camera decided during this lighting situation that my aperture needs to be at f4 and my shutter at 1/400 to give me a exposed image of ev=0. This image is what the camera thought was a perfectly exposed image.

ev = 0

Now the picture below is at ev = +2. Basically the camera knows that I want to over expose my shot by 2 f-stops. So the camera changed the settings to f2.8 and shutter speed 1/200. Increasing the aperture diameter and increasing how long the shutter stayed open made more light able to hit the sensor causing an over exposed image. Compare it to the picture at ev=0. One can see that the picture at ev=+2 is over exposed compared to ev=0.

ev = +2

Now lets look at the opposite. Exposure, ev = -2. Again the camera has to decide the aperture and shutter but now that you told the camera you want the picture under exposed it has set the aperture to f5.6 and shutter 1/800. This reduction in aperture width and reducing the time that the shutter is open caused less light to hit the sensor. Look at the below picture and see how it compares with the pictures at ev=0 and ev=+2. See how it is draker and more under exposed then the other pictures.

ev = -2