Depth of Field

In optics, particularly as it relates to film and photography, depth of field (DOF) is the distance between the nearest and farthest objects in a scene that appear acceptably sharp in an image. Although a lenscan precisely focus at only one distance at a time, the decrease in sharpness is gradual on each side of the focused distance, so that within the DOF, the unsharpness is imperceptible under normal viewing conditions.

Source - Wikipedia

So. Depth of field. A natural effect of using a lens that is often exploited and overdone. However, with portraits, and some pictures of inanimate things, it looks absolutely spectacular. it allows everything but what you want to be seen to be out of focus, thus focusing one's attention on the subject you are photographing.

Both of these shots of my niece were taken using a 70-300mm telephoto lens from a distance. Why? Because often if you hang back people are alot less aware of a guy with a camera in their face. You are more likely to capture moments in some circumstances that you would not normally capture. People and animals showing natural traits and emotions.

This one was taken with a 50mm. Once again, the only thing in focus is the subject. your eyes naturally go toward the thing that is in focus, not what is around it.