***Long story short: An experimental group is one that is actually being tested, a control group gets no treatment.
When you are performing an experiment, you are testing a certain hypothesis. You want the result of that experiment to comment directly on your hypothesis. For example, suppose you are testing a certain drug that is supposed to lower cancer number. You would want an experimental group to test the drug on, and a control group(a group you don't do anything to) to see how the cancer numbers would react without the drug. It is to ensure that the findings of the experiment are actually a result of your manipulation, not due to external circumstance.
