Exposure Control - The F/Stop
The smaller the f-stop the more light reaches the photographic plane (film, chip, etc) and vice-versa the larger the number the smaller the opening the less light that pass through the lens to form your photograph.
Wikipedia provides the following chart:
So, given the same shutter speed we now have a way to increase or decrease the exposure.
Another interesting consequence of f/stops is called depth of field. Depth of field is the amount of +/- distance of error you have when focusing. The more depth of field the more room for error you have in focusing (very helpful for moving subjects). However, depth of field comes with a cost and in order to get more depth of field you must use larger f/stops which often means slower shutter speeds, still with auto-focusing cameras being so prevalent now depth of field is less important.
Next lesson shutter speed.