During a scientific experiment, say testing a drug, there is more than one group to be tested. Both are treated identically (same conditions) but one is given a placebo (fake) so that the results of both test can be compared. The control group would be given the placebo. The control group allows the tester to determine if the drug (in our example) made a difference or had an affect. If both the tested group and the control group have the same result, they introduced drug did not have an affect.