What serves as a standard of comparison to evaluate the effect of the independent variable on the dependent variable?

The control group is a group of test subjects to which the independent variable is not applied. In other words, as a scientist, you would use the independent variable on all of the different study groups except for the control group. This is done for the exact purpose stated in the question: as a standard of comparison to evaluate the effect of the independent variable.

Example: you're testing to see if adding chili to a plant makes it grow faster. You will have two plants to test on. On ONE of them, you put the chili on (to see how the chili effects it). On the OTHER one, you DO NOT put the chili on (because you need the results of a regular plant to compare with the chilified plant). The plant without the chili is the control group.