Shutter speed and aperture generally control two things each.
Aperture controls the amount of light that enters the lens. The faster the aperture, (like F1.4) the faster shutter speeds you can use. There are a few side-effects, however. Fast apertures are good for night photos in which sharpness is a concern- remember that show shutter speeds will result in a shaken image, and you definitely don't want that. Fast apertures also minimize depth of field (DOF), which is the range of sharpness in a photo. The slower the shutter speed, the more DOF you get.
For example, if you're using a 50mm lens and focus in on a subject that's 3m away, but you also want everything beyond that sharp as well, try using a very slow aperture. Many lenses have a DOF scale printed on the distance scale, so you'll be able to see what I mean (the DA zooms do not have the scale, however).
A third factor with respect to aperture is image quality. This varies from lens to lens, but fast apertures may soften your image up a bit. The same goes for extremely fast apertures. Most lenses are the sharpest from F5.6 to F8, so try to stay in that range when taking normal shots. Also, P mode won't choose the slowest aperture unless you're shooting something very dark (or if you adjust the program line).
Shutter speed fundamentally also controls the amount of light that the film/sensor is exposed to. It's a bit more self-explanatory than aperture, IMO. The faster the shutter speed, the less light is allowed onto the sensor, but the sharper (less shaken) the image gets. If you use a very slow shutter speed, your pics may end up being blurry- and that's what SR tries to minimize.
Show shutter speeds can also be used to achieve soft effects. For example, if you set up your camera by a river and use a very slow shutter speed, the water will look very smooth. If you use a fast shutter speed, you're likely to be able to see every tiny droplet
I hope this helps!
Adam
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