Two hypoetheses are utilized in any experiment. These are used to explain the results. An alternate hypothesis specifies that the independent variable (treatment) is responsible for the differences in the outcome of those studied. This alternate hypothesis can be directional or nondirectional in nature and is specified in the wording of the hypothesis. The null hypothesis is the opposite of the alternative hypothesis. The null hypothesis states that the independent variable had no influence upon those studied. If the null hypothesis is proves false, then the alternative hypothesis is said to be true.
Hypothesis testing utilizes a decision rule to evaluate the results. This rule is designed to eliminate the possibility that the outcome of the experiment happened by chance and not due to the manipulated variable. This is termed the alpha level and is set at the start of the experiment at the .05 or .01 level. If the obtained probability is less than that level, then the null hypothesis is rejected and the independent variable is said to have had an impact. If the obtained probability level is greater than the alpha level stated at the start, then the null hypothesis is retained, indicating that the variable manipulated had no impact on the experiment,
The following website gives another view of these definitions.
http://www.cas.lancs.ac.uk/glossary_v1.1/hyptest.html