ISO Speed	How sensitive the sensor is to light
	Aperture (Lens Opening)	The amount of light that reaches the sensor
	Shutter Speed	How long light reaches sensor

It also affects how your picture "looks" as your selection of lens aperture affects Depth of Field - something that gives you "creative control" over you pictures.

Older apertures are set with a band on your lens.  There is usually a band around the lens with the aperture numbers on it.  This band rotates and there is usually a white "dot" that indicates which aperture you've selected. Newer cameras don't have this band and the aperture is usually selected with a dial on the camera body.

The lens aperture is referred to as the f-stop which is rated as a number.   

Lens openings are counter-intuitive.  That is, the larger the number, the less light it allows thru the lens.  As you can see by the chart below, the aperture setting of f 2 allows a lot of light to pass, while an aperture of f22 doesn't allow much light at all to pass thru.

These numbers are pretty much a "standard".  Almost every lens will have an f5.6, f8, etc. Although the f-stops are almost always rated the same way (f2, f2.8, f4, f5.6, f8, f11, f16, etc.) this is a continuous scale.  That is, you could always select f10 by adjusting the dial so the white dot is somewhere between f8 and f11.  You could set it for f17.6, f3, or anything in between the largest and smallest f-stops (assuming you could adjust the dial that accurately!).

How much light are we talking about?
The amount of light each sequential f-stop permits to reach the sensor is either double or half, depending upon the direction. For example...

Technically speaking...
The aperture value is arrived at by measuring the focal length of the lens divided by the physical diameter of the aperture - i.e., a 50mm lens with a 9mm aperture = f5.6 (50/9=5.555).  Thus you can see, to get an aperture of f4 on a 50mm lens, you'd need a lens that was at least a 12.5mm aperture (physical diameter of the lens) while a 300mm lens would require a 75mm aperture to achieve an f4 f-stop - thus you can see you'd need a lot more glass for a 300mm lens than a 50mm lens!

Depth-Of-Field
If you use a LARGE lens opening (i.e., F2.8, F4, etc.) you will cause a small portion of your picture to be in focus (good for portraits).  If you use a SMALL lens aperture, you will cause your picture to have a larger range in focus (good for landscape pictures). I won't get into Depth of field here as it's covered in another section, but be aware, that this is the primary reason for caring what your aperture is. You can't actually SEE the depth of field because when you view thru your SLR, you are actually viewing thru the lens at its widest aperture - otherwise it might be too dark to see anything! 

Exposure Modes
Automatic cameras often have several exposure modes: Program mode, Aperture Priority, Shutter priority and Manual.

Aperture Priority means that YOU select the lens aperture and the camera's internal meter determines what shutter speed to use for the correct exposure.  You want this mode when you want to control the aperture to achieve a certain depth of field result.

Shutter Priority means that YOU select the Shutter speed and the camera's internal meter determines what aperture to use for proper exposure.  You lose control over your aperture this way and can't control your depth of field - this is good when you care more about your shutter.

If you use your camera in PROGRAM mode your camera determines both your aperture and shutter - this is for when you just want to take pictures without thinking about things like depth of field or shutter speeds.

In MANUAL mode you gain full control by setting both your aperture and shutter.  You'll need a light meter - either one in your camera or a hand-held meter to determine what your shutter and aperture should be set at.