Fundamental Lessons in Photography - Understanding Exposure Like a Professional
Plainly stated, exposure is a state of vulnerability. More specifically and when thinking about photography, exposure is the amount of light being reflected off your subject and falling onto your camera's image sensor. It stands to reason that the more light which you allow into your camera, the more saturated with light your image sensor would become. The converse is also true in that if you limit the amount of light entering your camera, you'll experience a resultant decrease in exposure.
An uncomplicated analogy to explain exposure is to imagine light as being random particles or rays and your aim is to trap one thousand of these tiny particles into a small black box. You get faced with two scenarios, firstly one in which you find yourself on the surface of the sun! To trap a thousand particles or rays there, your common sense would tell you to open the box for a very short time to let your required quota of particles inside your box. No problem, there is light everywhere so your box gets filled quickly. Secondly, place yourself on the dark side of the moon where you are well shielded from the suns influence. In order to fill your box you would need to take the lid off the box for a very long time if you wanted to stand any chance of catching these elusive rays. You find yourself keeping the box open for a considerable time and you still may not necessarily meet your one thousand light particle quota.
Let's transpose the above analogy into photographical terms. Replace the word box in the above paragraph with the word camera, and replace the word lid with the term shutter. On the suns surface you would logically opt for a very fast shutter speed (in the order of milliseconds) to try to limit the number of light particles or rays entering your camera and falling onto your image sensor, because, too many light particles will unquestionably over expose your photograph and ruin it. Now going back to the moon, a very slow shutter speed in the order of minutes or hours would ensure that you do not under expose your image sensor to light rays, and ultimately produce a photo which is too dark.
It's important to remember that exposure and aperture go hand in hand with one another and that one can never consider either in isolation of the other.