For example, a 2 megapixel cam can take pics at 1600 x 1200 max resolution. That's 4:3 ratio. But labs usually process prints at 3:2 ratio, so you must crop it down to 1600 x 1067. If you're using PS, there's the crop tool.
If you don't crop, the lab will either do it for you, or you'll get...