A control group is used in many experimental research studies. It is used as a contrast group to a 'treatment' group which receives some form of treatment for a shared condition or problem. A control group is chosen with the same demographics and characteristics as the treatment group but receives no 'treatment' and is used as a comparison.
A control group helps ensure that any effects from the treatment aren't a result of the .
For instance, a scientist is researching clinical depression. He recruits a number of individuals that have been diagnosed as having depression. These people would be separated into two groups. The treatment group would receive medication and counseling, while the control group receives counseling and a placebo (rather than 'real' medication). At the end of the study the results of the study would be evaluated to see which group made better progress; the medicated group or the unmedicated group.