While there are several important considerations that go into creating a good experiment, the essential parts are the treatment and the control.
The experimental treatment is the thing that you are testing. It is the condition that is manipulated, and the act of doing so it what creates the experiment in the first place. The treatment generally consists of the independent variable, which is the variable that is being manipulated by the experimenter, and the dependent variable, which is observed or measured to see if it is responding to the independent variable.
The control is a repeat of the experiment in which the independent variable is not manipulated. The control's purpose is to set a baseline to which the dependent variable can be compared.