What are the factors that are tested by being varied by the experimenter?

The control variable never changes. It is constant.

The independent variable is changed by the experimenter in order to produce a change in the Dependent variable.

The independent variable is the "factor" that you are looking for.

For Example:
One might be testing the effects of mold growth speed on different types of bread.
The researcher would keep everything he was not testing constant (control variables.)
A control variable would be the mold type.

But the researcher would modify the bread type, thus finding which bread type mold grows fastest on. The bread type is the Independent variable.

The mold growth speed that is changed due to a change in the bread type (Ind. Variable) is thus the dependent variable.