Books have been written on the subject. Big books. Heavy books with lots of pages. Some of the pages have 
great photos as illustrations. I'll try a very basic explanation over the next few, or maybe many entries. As always comments and suggestions are always welcome. feel free to contribute.
There are three elements to photographic exposure. They are often referred to as a triangle, and the the three points of
the triangle are aperture, shutter speed and ISO. These three elements are interconnected so you need to
understand all three. In the next post we'll talk about aperture.