Summary
Exposure
Amount of light to which camera sensor is exposed.
Underexposure
It means that your photo will be darker than what it should be. It happens because sensor doesn’t get enough light for a correct exposure and image appears darker than it should be.
Overexposure
It is the opposite of underexposure and it means that your photo will be brighter than when it should be. It happens because camera sensor gets more light than it needs to take a photo.
Controlling exposure
To be able to change or correct the exposure, you have to use one of the following parameters as all three are responsible for correct exposure. All three parameters are bound together, so if you modify one, to maintain the same exposure, you will have to modify another one is the opposite direction.
Aperture
It control the amount of light that your sensor should be exposed to. It does it by opening and closing a ring within your lens.
Shutter Speed
It control for how long your sensor should be exposed to a light that reaches it through the lens
ISO
It control how clean your image is. By changing this parameter, your camera tries to copy pixels that are next to it and fill the image up with it, which sometimes create a grain.
Notes
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Exposure, quite simply, is the amount of light to which the sensor is exposed. We are all familiar with photos taken indoor without a flash and which appear too dark: they are underexposed, not enough light was allowed on the sensor. Conversely (though perhaps more rarely), we have also seen images too bright, with pure white in large areas: they are overexposed.
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However, what we will generally consider a good exposure is one with an even (but not necessarily linear, as we will see tomorrow) distribution of tones, from pure black to pure white, with no details lost to either shadows or highlights.
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all modern camera bodies include one or several light meters, whose role is to measure the quantity of light and give a guess of what the correct exposure should be. What you will do with this information will depend on the shooting mode you are using: in auto, the camera will simply set all the required parameters so that you can shoot without questions asked. Alternatively, it can let you set one or more parameters and fill in the remaining ones (aperture or speed priority modes), or it can let you do the whole thing yourself, mentioning how your settings compare to what it thinks you should do, but not acting on it (manual mode).
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Three, and only three, parameters control the quantity of light to reach the sensor. They are the usual suspects: aperture, shutter speed and ISO.
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Something that is extremely important to realize is that all three parameters are bound together. If you modify one and want to keep the same exposure, you need to modify another in the opposite direction. For instance, if you want to use a pipe with twice as much area (doubling the flow), you need to either cut the flow duration by half or use a twice as fine filter. Modifying a single parameter will result in a modification of the bucket content.
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Put your camera in A mode. What this does is let you control the aperture and set the shutter speed accordingly.