A null hypothesis
An alternative hypothesis
A test statistic which is calculated from sample data which should have different behavior when the null is false than when it is true (or the test is not helpful).
A rejection rule (from which a significance level could be obtained) or a significance level (from which a rejection rule is obtained)
A rejection rule says under what circumstances (what values of the test statistic) the null would be rejected. The significance level is the probability of rejecting the null when it's true (or the highest such probability if the null is not simple). Either will tell you the other.
A rejection rule says under what circumstances (what values of the test statistic) the null would be rejected. The significance level is the probability of rejecting the null when it's true (or the highest such probability if the null is not simple). Either will tell you the other.