John Venn was a British mathematician and philosopher born on 4th August 1834 in Hull, Yorkshire.  He is famous for coming up with Venn Diagrams in 1881. They were a representation of the relation between sets using circles within circles.
What is a Venn Diagram
A Venn diagram uses overlapping circles or other shapes to illustrate the logical relationships between two or more sets of items. Often, they serve to graphically organize things, highlighting how the items are similar and different.
Types of Sets
Two-Set Diagrams
The most basic Venn diagrams feature two overlapping sets:

Examples include:
A video review