The exposure of a photograph takes into account three things: how long or short the shutter is open, how large or small the aperture is, and how sensitive to light the film or digital sensor is. Those three things will cause the image to be correctly exposed, overexposed or underexposed.
Most cameras (and pretty much all cameras made since the 1970's) have a built in light meter. A light meter reads the amount of light in the scene--and takes into account what shutter speed, aperture and ISO you are planning to use--and determines what would make a correctly exposed picture.
Some light meters take into account the enteire scene, some just the center of the scene, and others somewhere in-between. In many modern cameras you can choose how the light meter reads the scene.
Most modern light meters are pretty accurate, and with digital cameras it's easy to make on-the-spot adjustments as needed.
If you've picked a shutter speed of 1/125, an aperture of f11, and an ISO of 200, the light meter might (for example) look at the scene and tell you that you will underexpose the image by two f-stops. You would then need to adjust the settings by two f-stops to correctly expose the photograph.
You can do that by adjusting the shutter speed, the aperture, or the ISO, or a combination of the three. You could change the shutter speed from 1/125 to 1/30. Or you could change the aperture from f11 to f5.6. Or you could change the ISO from 200 to 800. All three of those would accomplish the same thing: increase the amount of light by two "stops".
Let's say you want keep the ISO at 200 to avoid grain or digital noise. We'll also say that you're willing to drop the aperture to f8 but not to f5.6 because of depth-of-field. That means the shutter would need to be adjusted to 1/60.
That's how shutter speed, aperture and ISO work together: they determine the exposure. Each change to any of the three (except for intermediate changes that some cameras offer) will half or double the light.
It takes a little thought, but it's a lot better to control the photograph so that it is how you want it than let the camera decide for you and leave it to chance.
I won't go into too much detail, but film and digital sensors have a limited range of exposure. At some point details will be lost in the shadows, highlights, or both. Black and white film has the largest dynamic range, while digital sensors and slide film have the smallest dynamic range.
It's important to understand that you are working with a limitation. Many scenes will have a dynamic range that exceeds the capabilities of your film or digital sensor. You'll have to decide if you want details in those highlights or in those shadows, because you can't have both. You may not be able to expose everything correctly, so you will need to adjust the exposure to capture what you desire. For example, you may want details in the highlights, but don't care if the details in the shadows are lost, so you adjust the aperture from f11 to f16 to keep the highlights within the dynamic range of your medium.
You also could decide that you need to decrease the dynamic range of the scene by adding light with a flash or studio equipment. Sometimes a little fill-flash is all that is needed.
Or you may decide to wait for better lighting conditions to occure before you push the shutter release button. This is an option that is not exercised enough in our instant-gratification society.
Sunny 16 Rule
Not really a rule, but a pretty good starting point to determine exposure should you find yourself without a light meter. The "rule" says to use f16 on a sunny day, and have the shutter speed close to the same number as your IS0. For example, f16, ISO 100, shutter speed 1/125. You could then make adjustments as necessary to get the shutter or aperture where you want it for movement or depth-of-field (for example, if you wanted f5.6, and your ISO is 100, then set your shutter speed to 1/1000).
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It takes a little thought, and note-taking may help, but before long you'll understand exposure pretty well. You'll make judgements on what aperture, shutter speed and ISO to use quickly and with confidence. However, it does take practice, so be sure to learn by doing. In other words, grab your camera and start photographing.