The paired t-test is useful for analyzing the same set of items that were measured under two different conditions, differences in measurements made on the same subject before and after a treatment, or differences between two treatments given to the same subject.
For example, managers at a fitness facility use a paired t-test to determine whether their weight-loss program is effective. If the mean of the differences is different from 0, the managers use the confidence interval to determine whether the difference is practically significant.
If you have two samples of independent observations, use a 2-Sample t instead. For more information, go to How are dependent and independent samples different?.