Exposure is the quantity of light collected by the matrix in your camera during a single shot.

If the photo is exposed too long the photograph will be too bright. If the photo is exposed too short the photograph will appear too dark. Almost all digital cameras today have light meters which measure the light in the given shot and set an the exposure automatically.

Most photographers depend on the light meters which is fine, but if you know how to control your exposure you can get more creative pictures.
The two primary controls your camera uses for exposure are shutter speed (the amount of time the sensor is exposed to light) and aperture (the size of the lens opening that lets light into the camera). Shutter speeds are measured in seconds and more commonly parts of a second. (1/3000 of a second is very fast and 15′ seconds is very slow). Aperture is measured in f/stops (a very wide aperture is f/2.8 and a very small aperture is f/21).

So why isn't there just constant shutter speed or constant aperture so that we would only have to consider one setting? The reason is that even though they both control the amount of light getting to the sensor they also control other aspects of the picture. Shutter speed for example can be used to freeze subjects in the air when fast shutter speed is used or it can be used to blur water with slow speed.

Aperture influences the depth-of-field, which determines what is in focus in the shot. Aperture can be used to draw attention to one subject by blurring the background with a wide aperture (low f/stop). Aperture can also be used to focus everything in a picture with a narrow aperture (high f/stop).

On most digicams today you can change the sensitivity of the sensor when collecting light which is called the ISO. The most common span of ISO speed is 100 to 2000. The higher the ISO speed the faster the camera collects light but it also adds more digital noise to the images than the low speeds.

For instance if you're trying to take pictures in scarece light without a tripod you might want to raise the ISO speed in order to get a picture that’s not out of focus. Most of the time you should keep low ISO speed if there is enough light, it makes a big difference though, when there isn’t.

The best way master shutter speed and aperture is to experiment with them.