Aperture:
Is a hole or an opening through which light travels
Focus:
Due to the optical properties of photographic lenses, only objects within a limited range of distances from the camera will be reproduced clearly. The process of adjusting this range is known as changing the camera’s focus.
ISO:
Film speed is the measure of a photographic film’s sensitivity to light, determined by sensitometry and measured on various numerical scales, the most recent being the ISO system.
Shutter Speed:
Is a common term used to discuss exposure time, the effective length of time a camera’s shutter is open.
[Source: Wikipedia]