The aperture is the opening in a camera lens through which light enters. It is controlled by iris blades that open and close to regulate the amount of light. Aperture size is measured in f-stops, with smaller f-stop numbers indicating larger openings that allow more light. The focal length of a lens determines the magnification and affects factors like depth of field and background blurring. Together, aperture and focal length settings control the camera's exposure and creative options.