Variables are the parts of an experiment that could change. However, scientists only want to change one thing at a time (so they can observe what happens as a result). The one thing they change is called the Independent Variable, or IV for short. The thing they observe or measure to see how (and if) it changes is called the Dependent Variable, or DV. Other things could possible change or be different each time an experiment is run, but scientists want to keep as many things as possible the same, so it will be a fair test! These things that are kept the same are called the Controlled Variables.
For a more complete explanation, check out the Resources Page on Variables.
We also today reviewed the difference between Observation & Inference.