A Venn diagram is a way of visually representing sets.

The rectangle represents all elements in a given collection. For example, the collection can be whole numbers. Set $A$ and $B$ will then be subsets of whole numbers. $$\begin{array}{rl}A& =\{1,2,3\}\\ B& =\{3,4,5,6\}\end{array}$$ Then, in the Venn diagram, the red circle represents $\{1,2,3\}$ and the blue contains $\{3,4,5,6\}.$ The overlap of two sets are all elements that they share. In this case it's $3.$

Note that the circles have the same size even though they haven't got the same number of elements. Thus, Venn diagrams are generally not size proportional. If they are, it's called an area-proportional or scaled Venn diagram.

The intersection of two sets is the elements that the sets share. In a Venn diagram, it's marked by coloring the part where the circles overlap.

The union of two sets is all elements in both sets. In a Venn diagram, it's marked by coloring both circles.

The symmetric difference of two sets is all elements either in the first or second set but not in both. Therefore, both sets are colored except the part where they overlap.

The relative complement is the elements that are only in one set and not in both. For example, the relative complement of A in B is all elements in only B.

The absolute complement is everything that is not in the given collection. In a Venn diagram, everything except the collection is filled.

It is possible to represent more than the relation of two collections in a Venn diagram.