To get the right exposure, there are three main components:
Shutter speed – Shutter speed is the length of time your camera shutter is open, exposing light onto the camera sensor. Essentially, it’s how long your camera spends taking a photo. This has a few important effects in how your images will appear.
Aperture – Aperture is a hole within a lens, through which light travels into the camera body. It is an easy concept to understand if you just think about how your eyes work. As you move between bright and dark environments, the iris in your eyes either expands or shrinks, controlling the size of your pupil. In photography, the “pupil” of your lens is called your aperture. You can shrink or enlarge the size of the aperture to allow more or less light to reach your camera sensor.
I.S.O -In traditional (film) photography ISO (or ASA) was the indication of how sensitive a film was to light. It was measured in numbers (you’ve probably seen them on films – 100, 200, 400, 800 etc). The lower the number the lower the sensitivity of the film and the finer the grain in the shots you’re taking.
This is the picture I took on the Canon Explains Exposure website
Matthew Parkins Film 