Between 1939 and 1940 Ansel Adams and Fred Archer developed the Zone System technique for proper exposure and development. The Zone System gained momentum after Ansel Adams published the book, "The Negative".
There are countless resources about the Zone System on the internet and, if you truly love photography, you will benefit greatly by learning it. Basically the histogram on your cameras image is a Zone System giving you the ranges from dark to light. Traditionally the Zone System breaks up the image in eleven zones. Zero being total black and ten being total white.
All of this is important to know because it is what allows you to see the Tonal Range and the Dynamic Range so you can understand how to create the depth of the story you are attempting to tell with your photograph. It allows you to see your picture from the inside at every level and not as an outsider looking through glass.
What is so brilliant about the Zone System is that even though it was developed for B&W sheet film, it works perfectly well with color and especially in a digital workflow when we are adjusting the tonal qualities of the image.
Here are some useful links if you want to know more about the Zone System.
Ken Rockwell's discussion - [link]
DPanswers Adapting the Zone System to Digital Photography - [link]
Wikipedia - [link]