Here is a summary of the main set operations with the following subsets $A$ and $B$ of $E$ $$A = \{1,2\},\quad B=\{2,3\},\quad E=\{0,1,2,3\}.$$
Operation Notation Meaning Complement Union Intersection $A'$ $A\cup B$ $A\cap B$ Not in In either or both In both $\{0,3\}$ $\{1,2,3\}$ $\{2\}$