We have defined a set as a collection of its elements so, if S is a set then the collection or family of all subsets of S is called the power set of S and it is denoted by P(S).

Thus, if S = a, b then the power set of S is given by P(S) = {{a}, {b}, {a, b}, ∅}

We have defined a set as a collection of its elements if the element be sets themselves, then we have a family of set or set of sets.

Thus, A = {{1}, {1, 2, 3}, {2}, {1, 2}} is a family of sets.