Aperture and shutter speed:

When you take a photograph, two things happen to let through the desired amount of light for your exposure.

The aperture ring (or lens opening) inside the lens opens to a specific size and the shutter opens and stays open for as long as the shutter speed has been set.

The size of this aperture ring opening is measured in f-stops, and depending on your lens, can be anything from f1.8 to f22 or f32.
The confusing thing about this is that the smaller the f-stop number, the larger the lens opening.

The shutter speed is measured in seconds or parts thereof, e.g. 4 sec, 2 sec, 1 sec, ½ sec, ¼ sec, 1/8^th sec, 1/15^th sec, 1/30^th sec, 1/60^th sec, and so on, up to 1/2000^th or 1/4000^th sec.

The difference between each f-stop and between each shutter speed and between each film speed (ISO) is measured as a "stop".
This is important to know if you have to start compensating for specific shutter speeds or lens openings, which is explained later.

Closely linked to your aperture, as well as the length of your lens, is depth of field.

In simple terms this means that the wider your aperture, the shorter the distance between two points that will be sharply in focus.

On long zoom lenses, this distance will even be shorter than on short lenses.

The following explanation will give you an indication of what depth of field is:

(Please not this differs from lens to lens)

For instance - on a 50 mm lens (old cameras) f22, an object that you focus on 1 meter away from you, will render everything in the image sharply in focus between 0.8 meters and 1, 8 meters. Everything closer to you than 0.8 meters or further away than 1,8 meters will be out of focus.

Using the same lens at f22, and focusing on an object 3 meters away from you, will render everything sharply in focus between 1,5 meters and infinity.

This distance that is sharply in focus is referred to as "depth of field".

When the aperture size is f4:
focusing on an object 1 meter away, the depth of field renders everything in focus between 0,95 meters and 1,15 meters.
focusing on an object 3 meters away, the depth of field renders everything in focus between about 2,7 meters and 3,5 meters.

Unfortunately you can’t just change the lens opening for this effect without compensating by changing the shutter speed too, so as not to over or under expose, but we’ll get to that.

ISO:

A third factor that also influences your exposure, is the film speed. Even though digital cameras do not use film, most modern ones have an ISO (or film speed) setting. These normally range from ISO 100 to ISO 800, but on professional cameras can also have some of the folowing: ISO 25, ISO 50, ISO 100, ISO 200, ISO 400, ISO 800, ISO 1600, ISO 3200. The difference between each ISO setting is also measured as one "stop". The faster the film speed (ISO 3200), the less light is needed, and the slower the film speed (e.g. ISO 100),the more light is needed.
Fast film speeds (ISO 800 to ISO 3200) tend to render grainy images and should only be used in poor light conditions, or when you want graininess to create a specific atmosphere.

The following diagram should explain the relation between aperture size, film speed and ISO more clearly:

Example:
Using the diagram, you can now calculate your exposure settings in specific circumstances. For instance, you want to photograph a sport event on a cloudy day. The reading you get is maybe an aperture: f8, shutter speed: 1/250 and your ISO is 100.

You know you need to shoot at 1/1000 sec. to freeze the action.

On manual setting, you set the shutter to 1/1000. That means you LOSE two stops of light.

Now GAIN back those two stops by opening up the aperture by two stops (f4)

At f4 you are scared that the depth of field will not be enough to get a sharp image, but you don't want to use a slower shutter speed and get blurred action, so you gain those two stops you need by upping the ISO: Set the aperture back to f8, (which is always a safe aperture) and rather gain the two stops by setting the ISO two stops faster to ISO 400 (if you have a digital camera). With a film camera you will have to use ISO 400 film.

Note: with your automatic or digital camera you can use shutter speed priority and set it to 1/2000^th , and the f-stop will follow automatically, but you will have to maybe set the ISO to 400.

2. Take an outdoor portrait (close-up) photograph of a model between 12:00 - 13:00.

3A. Between 16:30 and 17:30 on a clear day, take an outdoor portrait photograph of a model facing East (sun behind model) on automatic exposure.
3B. Take the same photograph using a fill-in flash.
3C. Take the same photograph by taking a reading off the model’s face and lock it, or by using the manual setting, aperture priority or shutter priority.

4. Aperture exercise:
4A. Place your model at a table in an evenly lit (but not dark) area with an object (like a vase with flowers) ½ meter in front of, but not obscuring, the model. Position yourself opposite the model. Use your camera’s manual or aperture priority setting and take a photograph with the model in focus but the flowers out of focus.
4B. Take the same photograph with the flowers in focus and the model out of focus.
4C. Take the same photograph with the model AND the flowers in focus.