[Ibby]

To expand on what steve is saying...
inside the lens is an aperture ring. It consists of a bunch of 'wings' that roughly simulate a circular opening, measured in f-stops and usually able to move between 3 (or 4.6) and 22 (or even 30 in high-end lenses); in OLD, old cameras, pre-automatic, one had to physically turn a ring on the lens which would open or close this hole. Now, the cameras do it automatically with motors. Then, secondly, inside the camera itself is the shutter. on most DSLRs and film SLRs the shutter can stay open for up to a minute (or 30 seconds, depending on the camera; my old Minolta only goes up to 20 seconds, while my Olympus digital can go up to 90 seconds I think) down all the way to 1/2000th of a second.

'But', you say, 'why would I need TWO variables there?' The answer is twofold. A smaller aperture, confusingly referring to LARGER f-stop numbers, results in a bigger depth of field. For example, at a wide aperture, like 4.6, the depth of field might only be 2 or 3 inches deep, meaning less of the picture will be in focus, while a small aperture, for example 22, would result in a much greater depth of field, meaning that background and foreground elements would be much clearer. Thus, if the intended effect is to have a very small amount of the shot in focus, like a macro shot, a wide aperture is used. And so, the shutter speed would change to reflect this; f.22 would require a longer shutter speed. This ends up taking us to the second point; the shutter speed. A longer shutter speed would be used to create affects of, say, motion blur, or used with strobe lights or repeated flashes to create the affect of multiple exposure; a short shutter speed, to 'freeze' action like at sports events. However, dark situations, like indoors or at night, would obviously require a long shutter speed AND a large aperture; this makes night/dark condition photography very problematic without a tripod or a powerful flash.