One of the least used but best features of your DSLR camera is the ability to select the point your camera uses to focus on a subject. This control allows for framing photographs creatively (e.g. using the rule of thirds) in the camera; thereby removing the need to crop later. The button, or menu icon used to distinguish AF-Point selection is a square with five points inside that make a plus sign (as seen on the images above).
Most mid-range DLSR cameras have about 9 focal points. In the image below, note hoe four of them (circled in red) correspond to the four intersections ideal for focusing the subject based on the framing rule of thirds. It is especially import to read you manual to understand if your camera has different methods of selecting, and using AF-Points; especially which points can be selected, and how they might work in concert with nearby points.
NOTE: When you select the AF-Point, light metering is based off of that point. (REFER TO LIGHT METERING)