“Aperture” is a physical relationship between the primary lens element and the distance to the image sensor.

This page explains things rather neatly, from a photography perspective:

Hope this helps!

The ‘f’ in f11 or f/11 is a reference to the focal length of the lens you are using. A f/1 lens would have an aperture diameter the same size as the focal length of the camera, an f/2 lens 1/2 the focal length, etc.

The reason that this relative value is used, is that this ratio represents the same light collecting power for a lens whether it has a short, medium or long focal length. So f11 on a long focal length lens has the same effect as f11 on a short lens.

Aperture is basically 1/2 of the exposure equation. Exposure is a trade of between aperture (how much light the lens allows in) and shutter speed (how quickly the shutter opens and closes to capture an image. Your example could have been shot a couple of different ways, F16 shutter speed 1/60sec, or f8 1/250sec. A larger aperture number, or f-stop, yields a smaller opening and less light on the film plane (sensor). To compensate and give a properly exposed picture, the shutter speed is decreased. Conversely if you open the aperture, a smaller f number, it allows in more light and the shutter can be snapped quicker. So how you use it depends on your situation. If you need to capture an action shot, you want the fastest shutter speed possible, so you open the aperture, aka set it for a smaller f number. You can also use a smaller f number if you want a shallower depth of field, which will blur your foreground or background and make your subject stand out more. If you want a greater depth of field, you can slow down the shutter and “stop down” or close the aperture, higher f number and have a very deep range of in-focus area. Say you’re shooting a waterfall and you want to get that really cool smooth effect on the water, and you want the whole scene as crisp as possible. Stop down the camera as much as humanly possible, even to the point of using a tripod and slow the shutter down considerably. The water will move while the shutter is open and give a blurred effect on only the water while the rest of the area is still and sharp. One of my favorite things is to shoot in aperture priority mode, where you set the aperture and the camera figures the shutter speed. You can vary the aperture in order to get the desired shutter speed, and on old 35mm cameras the aperture ring is right next to the focus ring, so you didn’t have to take your eyes off the viewfinder to adjust exposure. The rule of thumb on shutter speed is anything slower than 1/60 sec is too slow for hand held operation, so you adjust your aperture to give you a shutter speed greater than 1/60.