Exposure Values (EV)
Exposure value refers to the amount of light for a given exposure. It is denoted as a single number. Its value is subsequently used to calculate the correct combination of shutter speed and aperture at a given ISO speed of the sensor.
Starting point is a shutter speed of 1 second at f1, which is denoted as EV 0. Further up the scale a single exposure of EV13 could be ISO 100, f8 - 1/125s, or f11 - 1/60s.
It doesn't make any difference which combination you choose, as all variations at the same EV will ensure a correct exposure. Remember though, you are always balancing camera or subject movement against depth of field. This is because a change in aperture will require a change in shutter speed for an equal amount of light to reach the sensor or CCD. If you set the shutter speed one stop slower and aperture one stop smaller, exposure doesn't change, but you increase depth of field slightly and the possibility of blur.
Most digicams will calculate all these values for you automatically and select a combination of aperture and shutter speed to give a correct exposure.
Some more advanced digicams will let you choose the right combination manually, offering you the choice to overexpose or underexpose in a given situation.