A Venn diagram is a diagram that shows all the possible logical relations between a finite number of sets. This Venn diagram shows the relationships between four sets, A, B, C and D. The set A consists of all elements in the region where both circles overlap. Set B consists of all elements in the region where both circles overlap and set C consists of those elements in the region where either circle overlaps. The set D consists of those elements that are not contained in any region where both circles overlap.