There are three major types of t-tests utilized. There are one-sample, independent samples, and dependent samples t-tests. In a one-sample t-test you only have one group and you are determining if the mean of that group differs from a predetermined mean. Most of the times this test is used the null hypothesis is that mean equals zero. The independent samples t-test is used when there are two independent groups and you are determining if their means statistically differ. The dependent samples t-test is similar to the previous test in that you have two groups and are determining if there is a mean difference between the two. The only difference is that these two samples are dependent of each other. An example in which samples are dependent of each other is a pretest and a post-test score for a group of individuals. They will have two separate scores but since it is the same person reporting them they are dependent on each other.