An experiment looks at the affect of a variable on a system. This requires the selection and identification of the dependent and independent variables.

An example is the affect of different types of music on a plants growth:

The independent variable is the factor that is being tested. In this example it would be the type of music that is played.

The dependent variable is the expect outcome that will be measured to determine if your experiment was a success. In our example this would be measuring the height of the plant ten days after it first broke ground. 

The controlled variables are all other factors that may affect the experiment; the scientist must insure that the only real difference between the different trials is the independent variable. In our example we will want to ensure that no one plant has an advantage over another plant, so they must receive the same amount of light, the decibels of the music (how loud it is) must be the same for each type of music, they all use the same soil, etc…

A control experiment does not contain the variable: in our example this would be a test that does not include music, thus testing to see if music has any affect at all.