A Venn diagram shows sets inside a rectangle called the universal set, written $\mathcal{E}$. Each separate area is a region, so in a two-circle diagram you can talk about A-only, B-only, the overlap, and the outside region.

The intersection, written $A\cap B$, means "in both sets". The union, written $A\cup B$, means "in A or B or both".

The complement of $A$, written $A^{\prime }$, means everything in $\mathcal{E}$ that is not in $A$. If an item is either in $A$ or not in $A$, then the probabilities satisfy $P(A)+P(A^{\prime })=1$.