In biology experiments, a control group is a group of subjects that are not given the treatment being tested in order to serve as a benchmark for the tested group. The presence of a control group helps scientists rule out alternate causes for any observed results.
Negative control groups are designed to create a population of study subjects that exhibit no effects when compared to the treatment group. As an example, a group of people given placebo pills instead of pills containing the new drug during a medical trial is a negative control group. The control group is expected not to experience effects from the drug because the pills those individuals consume do not contain actual medicine.
A positive control group is designed so that the control group exhibits a specific expected effect. In a biological test to determine the level of a protein in a test sample, a positive control that contains a known level of the same protein is used as a comparison to show the experimental readings are correct. If the test fails to detect the protein in the control sample, this indicates that the experiment has failed.
Not all experiments use control groups. Field experiments in natural conditions don't include a control group because the conditions include too many factors to be controlled.