Definitions of Set Notation
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 $B$.