Imagine you have 2 sets of 100 dollar bills you collected in two different years. The first set, you collected in this year. The other set, you collected it last year. Now imagine you want to know which of this dollars bills from both sets were made in the same year. If you construct a bubble containing the bills from the first set, and a bubble containing the ones from the second set, then if you intersect them together. The part that both bubbles shares would be the dollars bills that were made in the same year from both sets. In that case, that is call a venn diagram.