The aperture is the size of the opening in your lens to allow light onto your sensor or film.The wider the aperature, the more light that comes in at any instant. Aperture is measured in f-stops. A higher number means a smaller opening, which means less light.
The shutter speed is how fast the shutter is open to allow light in. The longer the shutter is open, the more light that comes in over a period of time.
To get the same exposure with a higher f-stop (smaller hole), you need to leave the shutter open longer, thus you will need a slower shutter speed. You want to make sure the shutter speed isn’t too slow, because vibrations in your hand will affect the image. If you need to use a long shutter speed, you will want to use a tripod to stabilize the camera.
ISO is a measurement of film. A higher ISO number means it is more sensitive to light, and you can speed up your shutter speed if you use a higher ISO film. In digital cameras you obviously can’t change the ISO of the film, but digital cameras let you emulate ISO values. A word of warning: higher ISOs increase noise in pictures (or “grain” in film cameras).
Here is a neat utility that let’s you see first hand how aperature, shutter speed and ISO affect the quality of an image.