CAMERA EXPOSURE

A photograph's exposure determines how light or dark an image will appear when it's been captured by your camera. Believe it or not, this is determined by just three camera settings: aperture, ISO and shutter speed (the "exposure triangle"). Mastering their use is an essential part of developing an intuition for photography.

UNDERSTANDING EXPOSURE

Achieving the correct exposure is a lot like collecting rain in a bucket. While the rate of rainfall is uncontrollable, three factors remain under your control: the bucket's width, the duration you leave it in the rain, and the quantity of rain you want to collect. You just need to ensure you don't collect too little ("underexposed"), but that you also don't collect too much ("overexposed"). The key is that there are many different combinations of width, time and quantity that will achieve this. For example, for the same quantity of water, you can get away with less time in the rain if you pick a bucket that's really wide. Alternatively, for the same duration left in the rain, a really narrow bucket can be used as long as you plan on getting by with less water.

In photography, the exposure settings of aperture, shutter speed and ISO speed are analogous to the width, time and quantity discusse
d above. Furthermore, just as the rate of rainfall was beyond your control above, so too is natural light for a photographer.

EXPOSURE TRIANGLE: APERTURE, ISO & SHUTTER SPEED

Each setting controls exposure differently:

Aperture: controls the area over which light can enter your camera

Shutter speed: controls the duration of the exposureISO speed: controls the sensitivity of your camera's sensor to a given amount of light

One can therefore use many combinations of the above three settings to achieve the same exposure. The key, however, is knowing which trade-offs to make, since each setting also influences other image properties. For example, aperture affects depth of field, shutter speed affects motion blur and ISO speed affects image noise.

source : http://www.cambridgeincolour.com/tutorials/camera-exposure.htm