John Venn popularized the diagram we associate with his name, but he did not invent it. It was likely used at least a century before him, and probably long before that by anyone with a stick, a plan, and some dirt. And he most certainly didn’t call them “Venn diagrams” while he was alive, which would have been a pretty egotistical thing to do.
“You know what would explain this? A ‘Me’ diagram.”
They are related to a way of describing data sets called Euler diagrams, who are named after a guy named Euler who probably didn’t call them “Euler diagrams”. John Venn actually called his “Euler circles”. Each of these diagrams have a simple definition: A set of closed curves drawn in a plane (like on paper) whose spatial representation shows you how their data relate. They are a stunningly simple way to explain logical relationships using geometry, actually.
If the circles are too hard for you to draw, you could always opt for the deluxe five-ellipse edition:
And who says that freehand circles are impossible to sketch? There’s a whole world championship for that, featuring guys like this: