When it comes to photography one of the concepts that is most difficult for many photographers to grasp is exposure. There are 3 main areas of photography that determine the exposure: ISO, Shutter Speed, and Aperture.
The easiest way to think about exposure is as a perfect triangle that has equal sides and equal angles. If you change just one aspect of that triangle or exposure, it will no longer be perfect. This means that if you change one of the three areas of exposure you will need to make changes to the others as well in order to make the exposure perfect once again.
When discussing exposure, it is important to also discuss lighting. When a picture is taken the light will pass through the lens and then through the shutter before arriving at a sensor or the film frame. The lens and the shutter of the camera control how much light is allowed to get to the sensor or frame.
The aperture is the lens opening that can be used to control the amount of light. Larger openings will allow more light in. One way to think about the aperture is by comparing it to blinds for a window. Opening the blinds allows more light in and closing the blinds keeps light out.
The next aspect of exposure that needs to be considered is the shutter speed. The shutter of the camera determines how long the light that comes through the lens gets to hit the film or sensor of the camera. This time is expressed in fractions of seconds. Fast shutter speeds would be 1/1000th of a second. This speed would be able to freeze most action immediately. A slower shutter speed such as 1/30th of a second would cause a person walking to be blurry. The majority of cameras today offer a wide range of shutter speeds.
Controlling the amount of light that is allowed in using the aperture combined with the shutter speed work together in order to create exposure.
This is the final side of the triangle. The ISO is the light sensitivity of the image sensor or film. When using film cameras, the ISO is somewhat limited and you are locked into the same sensitivity for the entire roll of film. However, with digital photography, it is possible to change the sensitivity as often as you like.
When it comes to ISO the lower the number the more light that is needed in order to get the right exposure. For example, if the ISO is 100 a lot of light will be necessary to get proper exposure. However, an ISO of 800 would require little light.
As a photographer, it is important to always remember the connection between the ISO, aperture, and shutter speed. If one element is changed the other 2 will need to be changed as well in order to get the clear, sharp photographs that are desired.