Scientists use the Scientific Method as a systematic approach to discovery. First, a tentative explanation is made to explain some phenomenon, this is called the hypothesis. Second, the hypothesis must be tested by the process we call experimentation. Simplicity is important to try and avoid influences of variables.
If the results of the experiment support the original hypothesis, then it is accepted as true. If not, then the hypothesis is rejected.
If other researchers can duplicate the results of the experiment, then the hypothesis is widely accepted. A hypothesis that has gained high levels of confidence is considered a law or theory.