Scientists use an experiment to search for cause and effect relationships in nature. In other words, they design an experiment so that changes to one thing causes something else to vary in a way that the scientist can describe as a 'trend'. The most useful way to describe a trend is a mathematical one.
These changing quantities are called variables, and an experiment usually has three main kinds: independent, dependent, and controlled.
The independent variable is the one that is changed by the scientist. In an experiment there is only one independent variable. This is usually plotted on the X-axis of the graph that the scientist uses to display his/her results in.
As the scientist changes the independent variable, he or she observes what happens.
The dependent variable changes in response to the change the scientist makes to the independent variable. The new value of the dependent variable is caused by and depends on the value of the independent variable. For example, if you turn on a water tap (the independent variable), the quantity of water flowing (dependent variable) changes in response - the water flow increases. The more open the tap - the faster the flow of water. The number of dependent variables in an experiment varies, and there is often more than one.
Experiments also have controlled variables. Controlled variables are things that would have an effect on the dependent variable. S/he must be sure that the only thing affecting that variable is his/her adjustment to the independent variable.
So, controlled variables are quantities that a scientist needs to keep constant, and s/he must observe them as carefully as the dependent variables.
For example, if we want to measure how much water flow increases when we switch on a tap, it is important to make sure that the water pressure from the water supply (the controlled variable) is held constant. That's because both the water pressure and the opening of the tap valve have an impact on how much water flows. If we change both of them at the same time, we can't be sure how much of the change in water flow is because of the faucet opening and how much because of the water pressure.
Most experiments have more than one controlled variable. Some people refer to controlled variables as "constant variables."
At GCSE the AQA examining board expect you to know several terms relating to varibales. Click here to look at the list of 'practical science vocabulary' that you are expected to know.