A Venn diagram is a diagram that uses ovals to show how certain sets or categories are related. A  set is a collection of things. Figure 10.72 shows a Venn diagram relating the set of mammals and the set of animals with 4 legs.  The overlapping region represents mammals that have 4 legs because these animals fit in both categories: animals with 4 legs and mammals. 

Venn diagrams are not just used in mathematics. Some teachers use Venn diagrams in language  arts, for example, to compare the attributes of a child with the attributes of a story character, as  in Figure 10.73. 

The sets in a Venn diagram do not have to overlap. Figure 10.74 shows two other kinds of Venn  diagrams. The first shows a Venn diagram relating the set of boys and the set of girls. These sets do  not overlap. The other Venn diagram shows that the set of whole numbers is contained within the  set of rational numbers. This is so because every whole number can also be expressed as a fraction  (by “putting it over 1”). 

When representing three or more sets, Venn diagrams can become quite complex. In general, there  can be double overlaps, triple overlaps, or more (if more than three sets are involved). For example, Figure 10.75 shows a Venn diagram relating the set of warm-blooded animals, the set of  animals that lay eggs, and the set of carnivorous animals. There are three double overlaps and one  triple overlap. 

We can use Venn diagrams to show the relationships among the various kinds of special quadrilaterals. 

Beckmann, Sybilla. Mathematics for Elementary Teachers with Activities (p. 481-483). Pearson Education. Kindle Edition.