When using auto-exposure, the camera meters the light based on what it sees through the lens — typically at the center of the frame with either a single spot or some kind of broader footprint. The camera also auto-focuses through the lens, usually at center unless you tell it otherwise. This is fine and dandy for most situations, but I'm willing to bet that most of us have encountered a situation where you want your light reading taken from somewhere other than the center of the frame. You can recognize this situation when your photos turn out with extremely under-exposed or over-exposed portions, and it typically occurs during sunset photos or other high contrast situations.
One way to deal with this is to point the camera at the area you want to meter around and allow it to focus there, thus locking in the exposure. This usually works fine for the scenes where nearly everything is focused out to infinity, but if that's not the case you'll end up with a properly exposed photo that's out of focus. The way to get around that is to point the camera at the area you want to meter and lock the auto-exposure — most SLRs have this feature, but some may be easier to use than others. Once you lock the exposure, you can focus on whatever you want and the exposure will stay the same until you unlock it. Now you have a photo that's correctly exposed AND in-focus!
Read your camera manual if you're not familiar with this feature, and give it a try.