The difference between a control group and an experimental group is one group is exposed to the conditions of the experiment and the other is not.
An experimental group is the group in a scientific experiment where the experimental procedure is performed. This group is exposed to the independent variable being tested and the changes observed and recorded.
A control group is a group separated from the rest of the experiment where the independent variable being tested cannot influence the results. This isolates the independent variable's effects on the experiment and can help rule out alternate explanations of the experimental results.
While all experiments have an experimental group, not all experiments require a control group. Controls are extremely useful where the experimental conditions are complex and difficult to isolate. Experiments that use control groups are called controlled experiments.
There are two other types of control groups where the conditions the group are subjected to will cause predetermined results.
Positive control groups are control groups where the conditions guarantee a positive result. Positive control groups are effective to show the experiment is functioning as planned.
Negative control groups are control groups where conditions produce a negative outcome. Negative control groups help identify outside influences which may be present that were not unaccounted for, such as contaminants.