ISO stands for International Organizatoin for Standardiszation. It is bascially a system that establishes standards. The ISO standards are used when refering to film speed. The term film speed is really what we want to talk about when we talk about ISO. I think it is easier to understand and remember. Ah...the good ol film days!!
In optics aperture is a hole or opening in which light travels. The larger the hole, the more light is allowed through the lens. In this case the more light allowed through the lens to be exposed on your digital sensor.
Shutter speed is the length of time the shutter is open to allow the light being exposed through the lens to hit the sensor. If light is very intense in all likelihood it will need to hit the sensor for a very short period of time. Therefore you will want a fast shutter speed, maybe even something like 1/1000 th of a second. If the light is very low you may need to expose the sensor to light for a rather long time, maybe up to 3 seconds.
So lets talk through a couple examples.
Lets say I want to take some photos of nightime cityscapes with the city lights illuminating the streets of the city. I am not going to use a flash and I would like to have images of high quality with little or no grain. I set my ISO to 200. Based on what I have learned an ISO 200 is not very sensitive to light and will need to be exposed for a longer period of time. In all likelihood I will need a tripod.
ISO - 200
Shutter - 2 Seconds
F-stop - f/11
It is late afternoon and I am taking photos of animals at a zoo. Not only is is later in the day but the animal I am photographing is in a shaded area therefore I have limited light. Typically during the daytime hours I would choose a lower ISO like 100 but I am not so sure that is going to work today. When taking photos of animals you will want a little faster shutter speed since they move unpredictably and faster than you think. I dont want blurry shots so I will need a faster shutter speed.
I do want a faster shutter speed so I set it at 1/500th of a second. Although my ISO is sensitive, I have selected a faster shutter speed. The light will be exposed for a very short period of time so I will have to let more in through the aperture. Again, depending on the actual situation the f-number will vary. To follow through with the example I will say I set the aperture at f/4.
Shutter - 1/500th
F-stop - f/4