Inferences from Two Samples

Dependent Samples: Samples in which the subjects are paired or matched in some way. Dependent samples must have the same sample size, but it is possible to have the same sample size without being dependent.
Independent Samples: Samples which are independent when they are not related. Independent samples may or may not have the same sample size.
Pooled Estimate of the Variance: A weighted average of the two sample variances when the variances are equal. The variances are "close enough" to be considered equal, but not exactly the same, so this pooled estimate brings the two together to find the average variance.