Distinguish between an experimental group and a control group?

In a controlled experiment, there are two groups. The control group is a group that nothing happens to. The experimental group is the group that you subject to the variable with which you are experimenting. At the end of the experiment, you test the differences between the control group, for whom nothing happened, and the experimental group, which received the variable. The difference (or similarities) between the two groups is how your results are measured.
A control group is the group used for comparison in an experiment. One group receives the treatment that is being tested by the experiment; another group (the control group) has the exact same controlled environment, but does not receive this treatment. The effectiveness of the treatment can then be established by comparison with the control group.