for example, claiming that a new drug is better than the current drug for treatment of the same symptoms.
In each problem considered, the question of interest is simplified into two competing claims / hypotheses between which we have a choice; the null hypothesis, denoted H0, against the alternative hypothesis, denoted H1.
These two competing claims / hypotheses are not however treated on an equal basis: special consideration is given to the null hypothesis.
We have two common situations:

 The experiment has been carried out in an attempt to disprove or reject a particular hypothesis, the null hypothesis, thus we give that one priority so it cannot be rejected unless the evidence against it is sufficiently strong. For example,
 H0: there is no difference in taste between coke and diet coke
 against
 H1: there is a difference.
 If one of the two hypotheses is ‘simpler’ we give it priority so that a more ‘complicated’ theory is not adopted unless there is sufficient evidence against the simpler one. For example, it is ‘simpler’ to claim that there is no difference in flavour between coke and diet coke than it is to say that there is a difference.
The hypotheses are often statements about population parameters like expected value and variance; for example H0 might be that the expected value of the height of ten year old boys in the Scottish population is not different from that of ten year old girls. A hypothesis might also be a statement about the distributional form of a characteristic of interest, for example that the height of ten year old boys is normally distributed within the Scottish population.
The outcome of a hypothesis test test is “Reject H0 in favour of H1” or “Do not reject H0”.
We give special consideration to the null hypothesis. This is due to the fact that the null hypothesis relates to the statement being tested, whereas the alternative hypothesis relates to the statement to be accepted if / when the null is rejected.
The final conclusion once the test has been carried out is always given in terms of the null hypothesis. We either “Reject H0 in favour of H1” or “Do not reject H0”; we neverconclude “Reject H1”, or even “Accept H1”.
If we conclude “Do not reject H0”, this does not necessarily mean that the null hypothesis is true, it only suggests that there is not sufficient evidence against H0 in favour of H1. Rejecting the null hypothesis then, suggests that the alternative hypothesis may be true.
In a hypothesis test, a type I error occurs when the null hypothesis is rejected when it is in fact true; that is, H0 is wrongly rejected.
In a hypothesis test, a type II error occurs when the null hypothesis H0, is not rejected when it is in fact false. For example, in a clinical trial of a new drug, the null hypothesis might be that the new drug is no better, on average, than the current drug; i.e.H0: there is no difference between the two drugs on average.
The critical value(s) for a hypothesis test is a threshold to which the value of the test statistic in a sample is compared to determine whether or not the null hypothesis is rejected.
The critical value for any hypothesis test depends on the significance level at which the test is carried out, and whether the test is onesided or twosided.
The critical region CR, or rejection region RR, is a set of values of the test statistic for which the null hypothesis is rejected in a hypothesis test. That is, the sample space for the test statistic is partitioned into two regions; one region (the critical region) will lead us to reject the null hypothesis H0, the other will not. So, if the observed value of the test statistic is a member of the critical region, we conclude “Reject H0“; if it is not a member of the critical region then we conclude “Do not reject H0“.
The significance level of a statistical hypothesis test is a fixed probability of wrongly rejecting the null hypothesis H0, if it is in fact true.
It is the probability of a type I error and is set by the investigator in relation to the consequences of such an error. That is, we want to make the significance level as small as possible in order to protect the null hypothesis and to prevent, as far as possible, the investigator from inadvertently making false claims.
 The significance level is usually denoted by
 Significance Level = P(type I error) =
Usually, the significance level is chosen to be 0.05 (or equivalently, 5%).
The probability value (pvalue) of a statistical hypothesis test is the probability of getting a value of the test statistic as extreme as or more extreme than that observed by chance alone, if the null hypothesis H0, is true.
It is the probability of wrongly rejecting the null hypothesis if it is in fact true.
The pvalue is compared with the actual significance level of our test and, if it is smaller, the result is significant. That is, if the null hypothesis were to be rejected at the 5% signficance level, this would be reported as “p < 0.05”.
Small pvalues suggest that the null hypothesis is unlikely to be true. The smaller it is, the more convincing is the rejection of the null hypothesis. It indicates the strength of evidence for say, rejecting the null hypothesis H0, rather than simply concluding “Reject H0‘ or “Do not reject H0“.
A onesided test is a statistical hypothesis test in which the values for which we can reject the null hypothesis, H0 are located entirely in one tail of the probability distribution.
In other words, the critical region for a onesided test is the set of values less than the critical value of the test, or the set of values greater than the critical value of the test.
A onesided test is also referred to as a onetailed test of significance.
The choice between a onesided and a twosided test is determined by the purpose of the investigation or prior reasons for using a onesided test.
Example
 Suppose we wanted to test a manufacturers claim that there are, on average, 50 matches in a box. We could set up the following hypotheses
 H0: µ = 50,
 against
 H1: µ < 50 or H1: µ > 50
 Either of these two alternative hypotheses would lead to a onesided test. Presumably, we would want to test the null hypothesis against the first alternative hypothesis since it would be useful to know if there is likely to be less than 50 matches, on average, in a box (no one would complain if they get the correct number of matches in a box or more).
 Yet another alternative hypothesis could be tested against the same null, leading this time to a twosided test:
 H0: µ = 50,
 against
 H1: µ not equal to 50
 Here, nothing specific can be said about the average number of matches in a box; only that, if we could reject the null hypothesis in our test, we would know that the average number of matches in a box is likely to be less than or greater than 50.
 TwoSided Test
A twosided test is a statistical hypothesis test in which the values for which we can reject the null hypothesis, H0 are located in both tails of the probability distribution.
In other words, the critical region for a twosided test is the set of values less than a first critical value of the test and the set of values greater than a second critical value of the test.
A twosided test is also referred to as a twotailed test of significance.
The choice between a onesided test and a twosided test is determined by the purpose of the investigation or prior reasons for using a onesided test.
Example
 Suppose we wanted to test a manufacturers claim that there are, on average, 50 matches in a box. We could set up the following hypotheses
 H0: µ = 50,
 against
 H1: µ < 50 or H1: µ > 50
 Either of these two alternative hypotheses would lead to a onesided test. Presumably, we would want to test the null hypothesis against the first alternative hypothesis since it would be useful to know if there is likely to be less than 50 matches, on average, in a box (no one would complain if they get the correct number of matches in a box or more).
 Yet another alternative hypothesis could be tested against the same null, leading this time to a twosided test:
 H0: µ = 50,
 against
 H1: µ not equal to 50
 Here, nothing specific can be said about the average number of matches in a box; only that, if we could reject the null hypothesis in our test, we would know that the average number of matches in a box is likely to be less than or greater than 50.
via http://www.stats.gla.ac.uk/steps/glossary/hypothesis_testing.html#h0