APERTURE

   Aperture is the unit of measurement that defines the size of the opening in the lens when a picture is taken that can be adjusted to control the amount of light reaching the film or digital sensor. The size of the aperture is measured in F-stop. Aperture size is usually calibrated in f-numbers or f-stops. i.e. those little numbers engraved on the lens barrel like f22 (f/22),16 (f/16), f/11, f/8.0, f/5.6, f/4.0, f/2.8, f/2.0, f/1.8 etc.

   There's a simple, direct relationship between aperture and depth-of-field - the smaller the aperture, the more extensive the depth-of-field. So if you want to keep as much as possible sharp, you should set as small an aperture as possible - preferably f/16, or even f/22 if your lens offers it.
Depending on lighting conditions and your film stock, you may need to use a tripod or some other form of support at such small apertures, as the resulting long shutter speeds create a risk of camera-shake.

   If, however, you want to concentrate attention on just one part of the scene, and throw the rest out-of-focus, you should select a large aperture. Exactly how large this can be depends on the maximum aperture of the lens you're using. On a 50mm standard lens it will be f/1.7, f/1.8 or f/2, but on a standard zoom it will typically be f/3.5 or f/4.5.
For general picture-taking, when you want most of the picture to be in focus, you might want to set a middling aperture of around f/8 to f/11

Aperture at f/10

Aperture at f/5.0