Exposure is basically how light or how dark an image is. An overexposed image is washed out because too much light reached the sensor. An underexposed images is dark because too little light reached the sensor. So, controlling exposure is all about controlling how much light reaches the sensor. The unit of measure that we use for light in exposure is the stop. Whenever you add one stop of light, youre basically doubling the amount of light. When you take a way one stop of light, youre cutting the amount of light by half. Taking away two stops of light means cutting the amount of light to just ¼ of what it was (because the first stop cut it in half, then the second stop cut it in half again
like folding a piece of paper twice give you four grids). So, a stop of light is not an absolute value its a relative amount.
Lets use a cooking analogy:
In a bowl you have one teaspoon of sugar. When you add another teaspoon of sugar, its like adding a stop, because you have doubled the amount of sugar in the bowl. Okay, so now you have two teaspoons of sugar in the bowl. If you add another teaspoon of sugar in the bowl did you add a stop? No, you did not. Because, remember a stop is not an absolute value; Its not always going to be one teaspoon. If there are two teaspoons of sugar in the bowl, then adding one stop would mean adding two teaspoons to the bowl. So, now you have four teaspoons of sugar in the bowl. How much sugar do you need to add in order to go up by one stop? Four teaspoons is correct. So, now you have eight teaspoons of sugar in the bowl, reducing it by one stop means taking out half, or four teaspoons. So, now you have four teaspoons left in the bowl. How much do you need to take out to reduce it by two stops? The answer is three teaspoons (the first stop means taking out two teaspoons --leaving two teaspoons in the bowl--, and the second stop means taking one teaspoon out of the bowl). I hope youre with me so far.
There are three basic camera controls that you can use to affect exposure, or how much light reaches the sensor. Those controls are: ISO, shutter speed, and aperture. ISO is how sensitive the sensor is to light. Shutter speed is how long you allow light to hit the sensor inside the camera body. Aperture is the pupil inside the lens through which light passes and enters the camera body.
ISO, shutter speed, and aperture do more than just control exposure and how much light reaches the sensor. Those settings can also be used for creative control of things like noise, lighting ratios, depth of field, etc.. I wont go into detail about those creative and compositional variables here.
Lets start with one of the easiest controls to understand: ISO.
ISO differs from aperture and shutter speed in that ISO doesnt actually affect how much light reaches the sensor. ISO actually controls how sensitive the sensor is to the light.
Examples of ISO settings are:
50.....100.....200.....400.....800.....1600.....3200.....6400
There are more settings than those, but these are enough to get us started. As you can see in the above sequence, the numbers double as you go left to right. That doubling represents one stop. In other words, ISO setting 200 is twice as sensitive to light as ISO 100. You could also rephrase this as ISO 100 is half as sensitive at ISO 200. ISO 200 is four times as sensitive at ISO 50. ISO 400 is eight times as sensitive as ISO 50 (because each stop doubles the light). ISO 800 is sixteen times as sensitive as ISO 50. Got it? I say that this part is easy, setting numbers themselves tell you how many stops brighter or dimmer they are than other ISO settings.
Shutter speed controls how long the light coming through the lens is allowed to hit the sensor. Examples of shutter speed settings are:
1/250.....1/125.....1/60.....1/30.....1/15.....1/8.....¼.....½.....1.....2
In the sequence above, shutter speed gets slower (longer) as you go from left to right. As you go left to right, you add one stop of light at each setting. Shutter speed is similar to ISO in that the setting number doubles as the light doubles...roughly. I say roughly because in a few spots (between 1/125 and 1/60, and again between 1/15 and 1/8) the light does double, even though the number in the setting isnt exactly double.
Aperture controls how big the diameter of the pupil inside the lens will be, and, thus, control how much light reaches the inside the camera. Example aperture settings are:
f/32.....f/22.....f/16.....f/11.....f/8.....f/5.6.....f/4.....f/2.8.....f/2.....f/1.4
In the sequence above the aperture gets larger as you go from left to right. Each aperture setting allows one stop of light (double) more than the setting to its left. Again, another way to phrase it is that each stop allow half the light of the setting to its right. Now, aperture confuses a lot of beginners for two reasons. The first reason for confusion is that the f-numbers in the settings get lower as the physical aperture gets larger (wider). The second reason for confusion is that, unlike ISO and shutter speed, when you double or half the light, the f-number doesnt double or half; there doesnt appear to be a clear pattern in how the f-stops are numbered. The reason for this is that the f-stops are actually fractions, where the numerator is the focal length and the denominator is the diameter of the pupil inside the lens. You dont need to understand, or even remember, how or why the f-stops got their numbers. You just need to memorize them and dont try to apply any logic or pattern to how they were numbered. Also know that all lenses set to the same f-stop allow the same amount of light into the camera.
I should also point out that the sequences above for ISO, shutter speed, and aperture only show full stops. Your camera probably allows half-stop or third-stop increments between those full stops. I dont memorize those incremental stops; I just remember how many clicks of the dial they are from the full stops. Its probably best to forget about the incremental stops for now until you understand exposure using the full stops.
Exposure depends on a combination if ISO, shutter speed, and aperture. Exactly which settings will give proper exposure will depend on the scene and its lighting. A variety of setting combinations can give the same exposure. For example: a particular expose correctly at ISO 400, 1/60 sec, f/2.8. The following combination of settings will yield the exact same exposure: ISO 200, 1/15 sec, f/4. Lets go setting-by-setting to see how thats possible. The second shot used an ISO of 200, which is one stop (half the light of) the first shot (ISO 400). The second shot also used a shutter speed of 1/15, which is two stops greater (four times as bright as) the first shot (1/60). The second shot used an aperture of f/5.6, which is one stop below (half as bright as) the first shot (f/4). So, lets review the math
the second shot used an ISO that was one stop dimmer (-1), a shutter speed thats two stops brighter (+2), and an aperture thats one stop dimmer (-1) than the first. If you add -1, +2, and -1, you get 0 (neutral). So, the second shot, even though the individual settings were different, results in the same total amount of light reaching the sensor as the first. Therefore, the exposure is the same. So, again, there is no ONE combination of settings that will give proper exposure.
How do you know which combination of settings to use? If you understand the creative uses of shutter speed to control motion/blur or light ratio, and if you understand the creative uses of aperture to control depth of field and light ratio, and if you understand the creative uses of ISO to control image noise, then you would start by picking the creative control that is most important for that particular shot. Since I havent explained the creative controls in this essay, Ill just throw out some safe general starting points. For ISO start at 400, for shutter speed start at 1/125, and for aperture start at f/5.6. From there, tweak the settings until you arrive at the proper exposure. You can use a meter (either a hand-held meter or the one built into your camera) to help.
Walking into a situation and reliably guessing settings using nothing but your eyes (without a meter) takes practice and experience. There are some general rules that can help you arrive at a close guess. The Sunny 16 rule is one guess. The Sunny 16 rule basically says that if youre photographing a subject thats in bright, full sun, then it will be properly exposed if your shutter speed is the nearest inverse of your ISO and your aperture is f/16 (hence Sunny 16). If you dont want to use f/16, then remember to offset the shutter speed and/or ISO by however many stops you adjust the aperture. For example, a subject may expose at ISO 400, 1/500 sec, f/16 according to the Sunny 16 rule, but it would have the same exposure at ISO 200, 1/1000, f/8. If you do the math youll see that the ISO and shutter speed were dropped by a combined total of 2 stops, and the aperture was increased by 2 stops, so the overall exposure didnt change.
Once you understand the Sunny 16 rule and how you can change the individual settings while maintaining the same exposure, you can use the Sunny 16 rule to make educated guesses for different lighting situations. If you do a Google search for Sunny 16 youll find an accompanying list of different lighting situations and how many stops under they may be from the Sunny 16 exposure. For example, a slightly overcast day may be one stop under the Sunny 16 exposure (so the shutter speed may still be the nearest inverse of the ISO, but the aperture would be f/11 one stop under f/16). An overcast day may be two stops under the Sunny 16 exposure. Open shade may be four stops under the Sunny 16 exposure. Indoors (varies) may be nine stops under Sunny 16 exposure. And so on. At least this will give you a starting point from which you can tweak your settings.
Usually youll want to keep your ISO as low as possible to avoid getting too much image noise (although I have been known to intentionally use a high ISO to add noise that will look like film grain when converted to black and white). Generally, youll want to use an aperture that gives you just enough range that all of the important stuff is in focus, and the distracting elements are out of focus. Finally, youll usually want a shutter speed that, if youre hand-holding the camera, will negate blur caused by camera shake. The rule of thumb for that is that the minimum shutter speed should be the inverse of the effective focal length (although that ideally assumes that you have reasonably steady hands, good stance/technique, and it also doesnt factor the benefits of image stabilization). There are general rules of thumb for the minimum shutter speeds for freezing different types of action (1/250 for normal walking, 1/500 for a child moving quickly, 1/1000 for running sports, etc.). Its also important to note that its not always best to freeze motion. Sometimes you want to use a shutter speed slow enough to introduce a little blur, giving a sense of motion. The popular example is using slow shutter speed so moving water (waterfall, running stream, etc.) looks silky-smooth rather than being frozen drops suspended in air. Often, in order to achieve a good exposure, you will not be able to use the shutter speed, aperture, and ISO that youd prefer. Welcome to the world of photography. Its all about compromise. Youll have to pick which creative control/setting(s) is most important and compromise on the rest (or take multiple exposures and fix in post). Sometimes youll need accessories, such as a tripod, neutral density filter, reflectors, artificial light, etc., to get around the challenges. This last paragraphs is touching on creative considerations that I said earlier I wouldnt get into, so Ill stop here (before this becomes a second volume).
If you have more specific questions feel free to ask.
I still haven't found exactly what I'm looking for, and I don't know how to explain it other than I read posts here referring to it - and those posts are always by people who've been shooting for many years going way back to high school. 
):
) but if I could, I could probably look it up somewhere. 
