So they call it the exposure triangle. Triangles are easy to draw. I put the three factors that contribute to exposure on the sides so I can illustrate the main effects that vary as you adjust each factor.
The exposure triangle shows the camera factors that you can change when you are adjusting your camera creatively. The triangle is notional. You cannot use it to construct an exposure calculation. One thing to note is that the shutter speed side of the trangle is actually more or less unbounded. Conventional cameras generally set a minumum exposure time of 1/8000 but maximum exposure time can be extended to many minutes with bulb mode.
There is a 4th factor. That factor is assumed constant for the triangle but of course it can be controlled as well. That is the amount of light in the scene. You can add light in several ways. Flash. Flood lighting. Going out in the midday sun (with the mad dogs and Englishmen.) You can subtract light as well. Turning down the lights. Going out at dawn, dusk or the dead of night. Placing a neutral density filter on the camera. If you throw on this 4th factor it becomes an exposure tetrahedron. Too hard to draw or make sense of.
Of course, it's just a pretty picture. An engineer would draw three axes. X, Y, and Z. Place each factor along an axis and then you can construct planes of correct exposure in the three-space defined by the cube. Hard to draw. Throw in the 4th factor, and it's impossible to draw.
Hey, someone liked this text so much they lifted it! gusindra.com/2011/08/16/exposure-triangle/
Here's a few notes on issues that come into play when dealing with the triangle:
1. If you stop your camera down heavily to f/22 or f/32 the light coming into the camera will decrease and so your exposure time will need to go up and blurring will become something you will need to deal with (generally by mounting the camera, or setting it down on a solid surface.) Something else will happen (a surprise!): every speck of dirt on your camera sensor will start to make itself visible on your photograph. This can be quite unpleasant. You can either pay someone to clean your sensor for you or you can get brave and buy a kit to do it yourself. Get brave... with the right gear it is safe and a whole lot cheaper.
2. Noise happens. You need to increase ISO to capture good shots in lower light, or to get good freeze with more depth of field (lens stopped down). If the subject is very interesting, forget about the noise. No one will see it. If you are still trying for the crisp noise free quality of a 100 ISO shot, you might try noise reduction software. Go easy though. NR software can make a shot look strange if you over do it. Experiment. Noiseware community edition is a nice one to try.