The experiment is designed so that only one variable is tested at a time. The aspect that varies between groups is called the experimental (independent) variable. There can only be one experimental variable per experiment.
The group that receives the experimental treatment is the experimental group. For example, if the effect of aspirin on heart disease is being investigated, the experimental group is the group of individuals taking aspirin. There may be more than one experimental group within an experiment. For example, one group may take a pill containing no aspirin, another group consumes one aspirin a day, while a third group may take 2 aspirins a day.
The control group (control treatment) provides the baseline to which the experimental group will be compared. The control group is often a group that receives water or a placebo, or a group that receives the standard treatment. For example, in the aspirin study, the control group would receive a pill that looks the same as aspirin, but contains no active ingredient (a placebo).
Controlled variables are quantities that a scientist wants to remain constant between the experimental and control groups. The controlled variables insure that only one experimental variable is tested per experiment. There are usually a number of controlled variables in a single experiment. If you are examining the effect of aspirin on heart disease, controlled variables might include that all participants are adults, are the same sex, have no other health complications, similar levels of blood cholesterol, and non-smokers. Controlling these variables insures that only the experimental variable is investigated and the reliability of the data obtained is increased.
The dependent variable changes in response to experimental variable. Simply put, the dependent variable is what is measured to assess the experimental outcome.
Results must be counted or measured in some way so that discrete information can be obtained. In the aspirin example, the number of people who develop heart disease is counted as well as the age at which signs of heart disease are apparent.
The experiment should be conducted a number of times to insure that the results are real. In addition, within an experiment there should be an appropriate number of replications. An experiment designed to investigate the effects of fertilizer on plants would use many plants in each group.