Imagine that you want to measure the rate of an enzyme reaction. This is done by using an assay for the appearance of the reaction products. Think about this before you look below. A reasonable expectation is that if you leave out the enzyme, no yellow color should appear. So, in this experiment, the control is a separate incubation omitting enzyme.
Controls serve two somewhat related purposes in an experiment.
The first is that they show that the system is behaving as one expects from previous knowledge, experience, and observation. This is necessary to allow an interpretation of the experiments. A good example is a very simple kind of recombinant DNA experiment called a transformation. The cells are normally sensitive to this antibiotic. In the presence of the antibiotic, only the cells with the plasmid can grow.