This is the foundation of photography. Without a good understanding of the Exposure Triangle, it will be impossible to grasp the rest of the principals involved.
There are three factors that control proper exposure.
Aperture (size of the lens opening),
Shutter speed (time of exposure)
ISO (sensor sensitivity to light).
Each one of these factors can be adjusted to properly expose a picture. In the same respect each one has its advantages and disadvantages. You will find no matter how you balance the triangle “You can’t have your cake and eat it too”.
The neat thing is some or all of them can be automaticly adjusted by your camera so you dont have to worry about it. This is where the other spots (modes) on the wheel (M, P, A, S) come in.
I guess because the two major camera brands just have to be different,
Nikon's symbols are:
M (Manual), P (Program), A (Aperture) and S (Shutter)
Canon will look like
M (Manual), P (Program), A/V (Aperture Value), and T/V (Timing Value)
They may be worded a little differently but they do the same thing.
In M (Manual Mode)
You control the Aperture and the shutter speed to get the proper exposure.
In P (Program Mode)
The Camera adjust everything to your pre-determined setting. This mode is almost like AUTO
In A or A/V (Aperture Mode)
You control the Aperture (Lens opening size) and the camera will automaticly set the proper shutter speed.
In S or T/V (Shutter Speed)
You control the shutter speed and the camera will automaticly set the proper Aperture.
Its about this time you would say WHY would I want to control anything if the camera will do it all in AUTO?
Well, to find that out you can go to the pages on Aperture and Shutter speed to see what affects those two legs of the trangle have on your finished picture.