If S is the set {A, B, C} then {A,C} is a subset of S. There are other sets that are also subsets of S as well; the complete list is as follows:
So the power set of S, written P(S), is the set containing all the subsets above. Written out this would be the set:
P(S) = { {}, {A}, {B}, {C}, {A, B}, {A, C}, {B, C}, {A, B, C} }