A variable is a part of the information involved in an experiment. The simplest definition is it is something that has an effect on something else. The effect will vary according to the influence it has on the subject.
A controlled group is necessary for all legitimate studies. It is a group that maintains a constant level of influence from different variables. The control group should attempt to replicate the natural phenomenon. Variables that are constant among the control and experiment groups are termed controlled variables.
The independent variable is the one difference introduced into the experimental group. The experimental group outcome is independent of the control group because of the difference. The independent variable should be limited to a single difference to eliminate crossover contamination.
The dependent variable is the observation being measured. It is dependent upon the independent variable. By reducing the independent variable to one, the dependent variable can be assumed to be caused by the one change.
To better understand the interaction let's construct a realistic experiment. A scientist wants to determine if plants respond to music. A control group is constructed where the plants are not subjected to music. The soil, plant type, water, and sun exposure will all remain the same for every group. These are control variables. An experimental group will be given the independent variable of rock music. A second experimental group will be given the independent variable of classic music. After a prescribed period, the plants will be measured for height. The plant height is the dependent variable because it depends on the conditions it was subject. The scientist can assume the differences, if any, are related to the music exposure. To truly verify the results, the experiment would be repeated. Repeating experiments is done to ensure the difference is not a fluke.