Power set of a set S is the set of all subsets of S including the empty set. The cardinality of a power set of a set S of cardinality n is 2n. Power set is denoted as P(S).
Example −For a set S={a,b,c,d} let us calculate the subsets −

Subsets with 0 elements − {∅} (the empty set)
Subsets with 1 element − {a},{b},{c},{d}
Subsets with 2 elements − {a,b},{a,c},{a,d},{b,c},{b,d},{c,d}
Subsets with 3 elements − {a,b,c},{a,b,d},{a,c,d},{b,c,d}
Subsets with 4 elements − {a,b,c,d}   _{Your Comment}