The experimental groups are going to be the groups of subjects that are being given a treatment.
The control group is a group that will not be given a treatment.
***It's smart to use a control group in an experiment because it acts as important comparison to the experimental groups and helps you rule out the possibility of other factors having caused the observed effects that you see in the experimental groups.***
For example, let's say you want to test medication A and medication B for treating the common cold. To do this you take 300 sick individuals and assign 100 to Medication A, 100 to Medication B, and 100 to a placebo (fake medication, just a sugar pill which has no medication in it...but they don't know that...they think they're getting a medication).
The subjects assigned to medication A or B are part of your experimental groups, and the subjects assigned to the placebo/sugar pill are your control group. If anything happens to your control group, for instance, let's say their colds get better, then it's hard to say that any of your medications actually worked, because even the people that weren't given a real medication got better. Oppositely, if the subjects of the control group don't get better and some subjects in the experimental groups (Medication A or B) do get better, then you have a much stronger case for that medication being effective.
