The aperture is the opening in a lens through which light passes to enter the camera. It is expressed in f-numbers like f/1.4, f/2, f/2.8 and so on to express the size of the lens opening
Shutter Speed
The shutter speed is the unit of measurement which determines how long shutter remains open as the picture is taken. The slower the shutter speed, the longer the exposure time. The shutter speed and aperture together control the total amount of light reaching the sensor
ISO
In digital photography, the ISO measures the sensitivity of the image sensor. The same principles apply as in film photography: the lower the number, the less sensitive your camera is to light and the finer the grain.