A maintained hypothesis or conjecture is called the null hypothesis and it is denoted as Ho. The researcher also defines an alternative hypothesis, which in practice is the state of nature that the researcher believes to be true.
The objective of hypothesis testing is to show evidence against the null hypothesis.
For instance, a researcher believes that the average weekly consumer expenditures on food in the state of Florida is less than $100. Therefore, the null hypothesis would be in this case that the average weekly consumer expenditures is at least $100. Using sample information the objective of hypothesis testing is to find evidence against this null hypotesis, lending support to the alternative hypothesis.
The typical analogy in the judicial system, when evidence is presented against a defendant, defines the null hypothesis that the defendant is innocent.
Additional information on the null hypothesis can be found in David Lane's text.