Sets are identified only by their members

This means that what makes a set distinct is its elements (as opposed to the order or reoccurance of those elements).

For example, sets $A$ and $B$ below are identical; but neither is identical to $C$. $A=\{1,2,2,3\}$ $B=\{3,3,2,1\}$ $C=\{2,1\}$