A good photograph is a piece of art by the artist who is a photographer. Every artistic photographer has three paint brusher to work with. Knowing how and when to use these three main brushes is the key to taking artistic photographs. This is something every professional photographer should have at his finger tips.
Controls the amount of light that enters the camera by changing the size of the hole. Bigger the hole, more light enters the lens.Also controls depth of field (the effect of creating background blur in a photograph). Bigger the hole, more background blur.
Controls how long the shutter remains open. The longer it is open, more light can enter.Can also used to create motion blur.
Adjusts the sensitivity of the camera sensor. The higher the number, the less light is needed to capture the image; but results in a more grainy image.
Just like in a painting, using the wrong brush or over using one brush can destroy the work. Same can be said for the three main controls in every photograph.