Do you take pictures of someone and even though you had the shot perfect in the viewfinder, when you snap the shot, the top of their head is chopped off? This might help you. There's a ton I don't know yet about photography but I do like to pass on what I do know.
I took this shot yesterday afternoon with a point and shoot camera. I was just playing with composition. Even though I framed it right, as you can see I chopped off the top of the maple tree buds.
This is often because of something called a "parallax error". This happens when using a camera with two lenses, one is used for taking the picture the other lens is the viewfinder. Usually the viewfinder is located above the camera lens. The two lenses see ALMOST the same thing but not exactly, as I've drawn below. The red lines are what the camera lens "sees" and the blue represents what the viewfinder lens "sees.
In the above picture, the top of the head won't be cut off. But, the closer you get to your subject, the parallax error has more of an effect. So in the shot below, the viewfinder would show you that the picture is framed correctly but when you snap the photo, the camera lens is going to cut our poor model's head off.
DSLR cameras eliminate this problem entirely because there is only one lens (single lens reflex). What you see through the viewfinder IS coming from the camera lens. A lot of point and shoot digital cameras also now use a through the lens (TTL) viewfinder. But cheaper digital cameras and any non-slr film camera will be affected by the parallax error.
How do you fix it? Just be aware and allow for a little extra room up top when framing a picture. Be very careful to adjust for it when taking close ups.