So with most cameras, half depressing the shutter "locks" the focus on whatever was in the center of the view finder. You then keep the shutter half pressed and re-compose the shot to your liking. Then press the shutter fully and snap your picture.
Why do this? Well you don't always want the subject in the center of the shot to be the focal point. The shots below show how the picture can change depending upon what you're focusing on. Give it a shot and see how you can create some unique images that you previously might have over-looked. (Click on it to see a larger image)
