The exposure triangle explains how the individual aspects of exposure, i.e aperture, shutter speed and ISO, affect the final exposure of the photo. It's a useful way of describing the relationship between the three aspects of exposure. Each side of the triangle represents one of the three variables, aperture, shutter speed and ISO. Adjusting just one of these will will change the appearance of the photo based on your settings.
It's important to understand that unless you are in the Manual Exposure Mode, changing any of the three settings will not make the image darker or lighter. That's because in any of the semi auto modes (such as A/AV, S/TV, or P), the camera will automatically compensate by changing one of the other settings.
The aperture, which is part of the lens not the camera, controls the quantity of light entering your camera. Just like the pupil of the eye, the larger the aperture, the more light is let in. The aperture is set using what is known as F numbers, i.e f2.8, f4, f5.6, f8 and so on, and the smaller the number, the larger the aperture (only a small elite of quantum physicists know why this numbering system is so perverse!!). These numbers are sometimes referred to as a 'stop', and each stop allows in twice as much light as the previous one, so or example f4 lets in twice as much light as f5.6
The shutter speed, measured in fractions of a second, controls the duration of the exposure. Just like window shutters, the longer the shutter is held open, the more light is let in. The shutter speed is selected using the appropriate time value, for example 1/60th of a second, 1/125th, 1/250th, 1/500 and so on. Just like apertures, each speed value is sometimes referred to as a 'stop' and each stop doubles or halves the amount of light compared to the previous stop, e.g 1/125 of a second lets in twice as much light as 1/250th.
The shutter speed's influence on exposure is perhaps the simplest of the three camera exposure settings, it correlates exactly 1:1 with the amount of light entering the camera. When the exposure time doubles the amount of light entering the camera doubles.
The ISO speed determines how sensitive the camera is to incoming light, a lower ISO speed is always desirable since higher ISO speeds dramatically increase image noise. Generally speaking, the larger your camera's sensor the better it is at handling high ISO noise. ISO speed is usually only increased from its minimum value if the desired aperture and shutter speed aren't otherwise obtainable.
For the youngsters out there, image noise used to be known as "film grain" in traditional film photography, and the ISO was known as ASA. Common ISO speeds include 100, 200, 400 and 800, 1600, 3200, 6400, although many cameras also permit lower or higher values. Depending on the age of the camera, using an ISO speed in the range of 50-800 with compact and bridge cameras generally produces good low image noise, whereas with DSLR cameras, a range of 50-3200 (or higher) often still gives great results.
The burning question - Aperture or Shutter Speed priority
You can therefore use many combinations of the above three settings to achieve the same exposure, so which one auto exposure mode should you use, which is best?
This is very much a creative decision, since each setting also influences other image properties. For example, aperture affects depth of field, shutter speed affects motion blur and ISO speed affects image noise.