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In an experiment there are two variables; the Independent Variable (IV) and the Dependent Variable (DV). In the most basic sense, you need two variables because as a researcher, you want to be able to examine if something (a drug, a therapy, a teaching technique, whatever) has an effect on some participant (person, people, animals, etc.). To accomplish this, you need to have something to examine (and manipulate -- this is the IV); some variable of interest, as well as something to measure the effect the IV has (this is the DV).
Therefore, we can define the independent variable as the experimental variable or variable that is manipulated by the research and has some effect on the DV. If there is a change or effect, we may conclude that the IV affected the DV. The ultimate here is to establish that the IV caused the change in the DV (this is the magical "cause-effect"). As a quick example, if you want to study the effect of drinking 12 ounces of beer before an exam on exam performance, the beer would be the IV (we may have one treatment group whose participants drink the beer and one control group who does not drink the beer); the performance on the exam would be the DV.