Exposure

The exposure is the amount of light received by the film or sensor and is determined by how wide you open the lens diaphragm (aperture) and by how long you keep the film or sensor exposed (shutterspeed). The effect an exposure has depends on the sensitivity of the film or sensor.

The exposure generated by an aperture, shutterspeed, and sensitivity combination can be represented by its exposure value "EV". Zero EV is defined by the combination of an aperture of f/1 and a shutterspeed of 1s at ISO 100 (1). Each time you halve the amount of light collected by the sensor (e.g. by doubling shutterspeed or by halving the aperture), the EV will increase by 1. For instance, 6 EV represents half the amount of light as 5 EV. High EVs will be used in bright conditions which require a low amount of light to be collected by the film or sensor to avoid overexposure.

From the above it is clear that a certain exposure value can be achieved by a variety of combinations of aperture, shutterspeed and sensitivity. For instance if you are shooting at ISO 100 with an aperture of f/8 and a shutterspeed of 1/125s, doubling the shutterspeed to 1/250 (halving the exposure time) and reducing the f-number one stop to f/5.6 (doubling the aperture) will lead to the same exposure of 13 EV. Or if you double the shutterspeed to 1/250s (halve the exposure time) while keeping the aperture unchanged at f/8, you could double the effect of the incoming light by doubling the sensitivity to ISO 200, thereby keeping the EV constant at 13 EV. Note that doing so will increase noise levels in digital cameras and film grain in conventional cameras.

In automatic mode, the camera determines the optimal combination of aperture, shutterspeed, and sensitivity (4) based on the exposure value determined by the light metering system. A high EV indicates bright conditions, hence the need for high shutterspeeds, high f-numbers, and/or low sensitivities, to avoid overexposure. When you change the aperture in aperture priority mode, the camera will adjust the shutterspeed to keep the EV constant. In shutter priority mode, the camera will adjust the aperture to keep the EV constant.

See also exposure compensation.

Technical footnotes (for the purists):
(1) Strictly speaking, the term "exposure value" is used to represent shutterspeed and aperture combinations. An exposure value which takes into account the ISO sensitivity is called "Light Value" or LV and represents the luminance of the scene (2). For the sake of simplicity, as is the case in this article, Light Value is often referred to as "exposure value", grouping aperture, shutterspeed and sensitivity in one familiar variable. This is because in a digital camera it is as easy to change sensitivity as it is to change aperture and shutterspeed. Many digital cameras even offer an auto-ISO mode. Although sensitivity will not change the amount of light entering the camera, it changes the effect of it and is therefore a third variable that can be adjusted to achieve an exposure that matches what is measured by the camera's light meter. As stated in the article, changes in the sensitivity will affect the noise levels in the image.
Given the automatic metering systems in current cameras, the absolute EV value is less important than in the days when people were working with exposure tables. What is more important is to understand the effect of aperture, shutterspeed, and sensitivity on the exposure (and quality) of the image.
(2) There is also a variant, called "Brightness Value" or BV, used in the APEX system.
(3) When sensitiviy is adjusted, it is not the amount of light but the effect of it that is adjusted.
(4) In case the "auto-ISO" option is selected.

Source : http://www.dpreview.com