Types of Error
Type I error or alpha error
 A type I error or alpha error is when the the null hypothesis is rejected when it is true.
 The probability of committing a type I error is called the "level of significance" for a test and is often referred to as the alpha.
Type II error or beta error
 A type II error or beta error is when the null hypothesis is accepted when it is false.
Table 1: A diagram to illustrate the conditions when various types of error occur.

If H_{0} is true 
If H_{0} is false 
If H_{0} is rejected 
Type I error False Positive 
No error True Positive 
If H_{0} is not rejected 
No error True Negative 
Type II error False Negative 
Also See:
Chapter 10  More on the Testing of Hypotheses pages 174 in:

Phillips, J. L. 2000. How to think about statistics. W. H. Freeman and Co. New York. 202 pp. ISBN 0716736543
Chapter 6  Introduction to statistical Hypothesis Testing pages 8283 in:

Zar, J. H. 2007. Biostatistical Analysis. PrenticeHall, Inc. Englewood
Cliffs, New Jersey. 718 pp.
