This is borrowed from: http://www.bdebaca.com/blog–tutoriales/profundidad-de-campo & translated from Spanish by Google. Many thanks to the author:
The depth of field, also known as DoF thanks to the internet as it is an abbreviation (Deep of Field), refers to the number of planes that we focus both forward or backward point we are focusing on.
Well to explain more clearly put the following example, where my focus point is on the yellow doll hair black. As increasing depth as I close the diaphragm I will also focus on gaining dolls that are near yellow wrist. Oppositely while the diaphragm and opens the farthest dolls are losing sharpness gradually until only the wrist in focus is clear.
How to control the depth of field?
The depth of field can be modified in two ways, one is the aperture, the more we open the diaphragm will get less depth of field and viciversa, if we close the diaphragm obtain greater depth of field.
The other way is with the focal length, the lower the focal length of our lens will be easier to obtain images with great depth of field.
It is curious that in social networks of photography many people say, excellent DoF to photographs that have virtually no depth of field, when it should be the other way around. Some time ago I took a picture of the stars, using very shallow depth of field because all stars are in the same plane (in this case infinity) but has little depth of field, all the stars that are on the infinite plane (ie all) will in focus).