As noted in the discussion of Null Hypothesis (Biostatistics Text), the Null Hypothesis (H0 )is there is no difference in the parameter being studied.
Type I Error occurs when H0 is rejected but, in fact, it is true and should NOT be rejected. The probability of a type I error is known as α and is reported by investigators as the p Value (Biostatistics Text).
Type II Error occurs when you do NOT rejected H0 (and accept the alternative hypothesis H1) but, in fact, HO is true and should not be rejected. The probability of a type II error is known as β.
The interaction of α and β are an important part of Hypothesis Testing (Biostatistics Text)