An experimental group is a group of test subjects who were selected to participate in an experiment and were chosen (either randomly or deliberately) to receive the experimental condition. The experimental group are the active participants who are being observed, using a test medication, or taking part in whatever the experiment is measuring. This is in contrast to the "control" group who are not actively participating but who are being observed for comparative purposes.
For instance. if a pharmaceutical company is testing a new medication they would select two groups using the same criteria for eligibility (similar age, background, health, etc.) and would give one group the medication and then give the control group a placebo as a means of testing the efficacy of the medication. The group receiving the real medication would be the experimental group.