Basic set notation. Let $A$ and $B$ be sets.

 $x\in A$ means $x$ is an element of $A$ $x\notin A$ means $x$ is not an element of $A$ $\{x\ |\ P(x) \}=$ the set of all elements $x$ such that $P(x)$ is true

(The above notation is called set-builder notation. $P(x)$ is any property that is either true or false, depending on $x$.)

 $A=B$ is true when $A$ and $B$ have the same elements $A \cup B=$ $\{x \ |\ x \in A \mbox{or} x \in B \mbox{ (or both)}\}$ $=$ the union of $A$ and $B$. $A \cap B=$ $\{x \ |\ x \in A \mbox{and} x \in B\}$ $=$ the intersection of $A$ and $B$.