In experimental research, often participants are allocated truly randomly to an experimental group and a control group, enabling unmeasured or unknown variables to be taken into account and strengthening claims for internal validity.

The participants in experimental group are exposed to the independent variable such as a new drug, whereas, those in control group are not.

Example: Participants in experimental condition (the positive mood condition) perform some task and receive positive feedback on their work (i.e, praise). Those in the control condition would receive neutral feedback—comments designed to have no effect on the participants’ current moods.

Sources:
Research Methods in the Social Sciences, Bridget Somekh and Cathy Lewin (Click for eBook)
Psychology, Robert Baron (Click for eBook)