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Depending on the capability or your camera, either through dials or through menus, you may find different exposure modes.  They differ between camera manufacturers, but they share many similarities.  Because they are all linked to the same internal light meter in the camera, all except for Manual will give, or attempt to give, the equal exposure to any other mode; the difference between them is how they balance the values for Aperture, Shutter Speed, and ISO.

Basic Zone
Canon lists the following modes under Basic Zone. Basically all the photographer does is point and press the shutter button.

•Full Auto = Fully automatic mode where the camera takes care of everything.

Fully automatic modes for a particular Subject:
•Portrait mode
•Landscape mode
•Close-up mode
•Sports mode
•Night Scene mode 

Creative Zone
Canon lists the following exposure modes under Creative Zone. They are semi-automatic and manual modes that enable the photographer to take control of the camera to obtain the desired result.

•P = Program AE Mode
•Tv = Shutter-Priority AE Mode
•Av = Aperture-Priority AE Mode
•M = Manual Exposure Mode

P: Program AE
In this mode the camera automatically sets the shutter speed and the aperture. You do have creative control over such things as AF mode, drive mode, ISO, white balance and file size.

If you have a limited understanding of when to use slow/fast shutter speeds and small/large apertures, start by using the P mode. 

A creative tool that you can use in "P" mode is called Program Shift. Lets say that the recommended exposure for a scene is 125 8.0 (1/125 sec at f8). You decide to use a faster shutter speed to prevent the effects of camera shake. You turn the main dial and the camera now recommends 500 5.6 (1/500 sec at f5.6). Basically, the camera shifted the exposure values while still allowing the same amount of light to expose the sensor.

Tv: Shutter-Priority AE
In this mode you manually set the shutter speed and the camera automatically sets the aperture to ensure an accurate exposure.  Be aware that this mode can fail you.  It is possible to request a shutter speed from the camera that requires an aperture that the camera can not match. For instance if you are in a dark room and put in a shutter speed setting for 1/500 of a second, the camera does not have an aperture large enough to allow in enough light to get a correct exposure and your image will come out underexposed.

A creative use of shutter speed is using slow shutter speeds to blur an image and give a sense of motion. Conversely, fast shutter speeds freeze motion.

Shutter Speed Display
The shutter speeds from "8000" to "4" indicate the denominator of the fractional shutter speed. For example, "125" indicates 1/125 sec. Also, "0"5" indicates 0.5 sec. and "15"" is 15 sec.

Av: Aperture-Priority AE
In this mode you manually set the aperture and the camera automatically sets the shutter speed to ensure an accurate exposure.

The "standard" sequence of aperture values (f) is:

1 1.4 2.0 2.8 4 5.6 8 11 16 22

You may have noticed that your camera displays additional aperture values such as 7.1 and 9. The most important thing to remember about these numbers is that the smaller numbers (1.4, 2, 2.8, 4) represent large openings (apertures) in the lens and the larger numbers (11, 16, 22) represent smaller openings (apertures) in the lens.

When smaller f-numbers are used, less of the foreground and background are in acceptable focus. When larger f numbers are used more of the foreground and background are in acceptable focus.

The term depth of field is closely related to aperture values. Depth of field is defined as the area of acceptable focus that stretches from foreground to background. When photographing a person or a single flower you want that area to be narrow but when photographing a beach or a city/town that area should be as wide as possible.

M: Manual Exposure
This mode is best suited for professionals and persons who have a very good knowledge of when to use small and large apertures and slow and fast shutter speeds.

In this mode you manually set the shutter speed and the aperture to suit the lighting condition. One of the best ways to determine the correct exposure is to use a commercially available handheld light meter.

Alternatively, you can use the exposure level indicator in the viewfinder to help you get the correct exposure.