A control in an experiment typically refers to the control group, which is the group that is not being exposed to the element or factor being tested. The other group is the experiment group. The only difference between the two groups should be the experiment factor.
According to About.com, controlled experiments have several strengths and weaknesses. One of the strengths of having a control group is that it allows causation to be established. It also singles out individual determinative factors since all other factors are constant. However, controlled experiments tend to be artificial; that is, they are typically done in a laboratory and do not account for many natural responses. Therefore, controlled experiments need to be evaluated in order to determine the extent to which the artificial setting has influenced the outcome.