Rewrite in your own words:

"Experimental research requires, then, that the responses of at least two groups be compared. One group will receive some special treatment—the manipulation implemented by the experimenter—and another group will receive no treatment or a different treatment. Any group that receives a treatment is called an experimental group; a group that receives no treatment is called a control group. In some experiments there are multiple experimental and control groups, each of which is compared with another group.

By employing both experimental and control groups in an experiment, researchers are able to rule out the possibility that something other than the experimental manipulation produced the results observed in the experiment. Without a control group, we couldn't be sure that some other variable, such as the temperature at the time we were running the experiment, the color of the experimenter's hair, or even the mere passage of time, wasn't causing the changes observed."