The factor that is changed or controlled in an experiment is called the independent variable. An experimenter makes alterations to the independent variable in order to test the effects these changes have on the dependent variable.
A scientific experiment often contains a control group and an experimental group. The control group is isolated in such a way that the independent variable being tested won't have an effect on the group. In contrast, the experimental group is exposed to the independent variable, and the differences between the two groups are recorded. Setting up the experiment this way helps to isolate the exact effects of the independent variable and rule out alternate explanations of the results.