An experimental variable is something that a scientist changes during the course of an experiment. It is distinguished from a controlled variable, which could theoretically change, but the scientists keep constant.
For example, if a scientist is investigating how salt affects water, he would want to measure and record such details as the amount of water present, the amount of salt added, the temperature, whether or not he stirred it, and any reactions or changes that take place. However, the only one of these variables that should change is the amount of salt present. The amount of salt present would be the experimental variable. All of the other variables, such as the temperature, should be kept consistent and are designated as controlled variables. Experiments typically have many controlled variables, but only one experimental variable.
Experimental variables are crucial to the scientific process; without them, the experiment is pointless. However, the controlled variables in any experiment are equally important. It is imperative for scientists to control everything in the experiment that they can, except for the variable that he is testing. Without controlling all other possible changes, the scientist could not be certain what was causing the change in the experiment.