Understanding Depth of Field
The next concept to understand is depth of field. This represents the portion of the photo that is in focus. A shallow depth of field or DOF has the subject in focus but the foreground and background is blurry, and a wide depth of field has most, if not all, of the photo in focus.
There are two things that affect depth of field: focal length and aperture.
The longer the focal length, i.e. the millimeters on the side of the lens, the more shallow the depth of field will be. This is why when you see telephoto shots of athletes or wildlife, the subject is in focus but surrounding area is not. This is actually a really good feature because the photographer generally wants to eliminate distractions from the main subject.
However the opposite is true with wide angle lenses, which generally have a lot more of the frame in focus. In most cases, this is desirable with wide angle shots.
Before reading this section, make sure you understand what the aperture is.
The larger the aperture, the shallower the depth of field. One of my favorite lenses to photograph flowers is a simple 50mm 1.8f lens. At f/1.8, the depth of field is very shallow; in fact, sometimes not even the entire flower is in focus. This makes for some interesting effects such as blurring out boring stems, the ground, and other distractions.