What does "strict subsets of" mean? How is it different from "subsets of"?

Answer 1

Strict subset means that the subset can not actually be equal to the set it is a subset of.

Answer 2

A "strict subset," or more commonly "proper subset" simply means a subset that is not actually equal to the set of which it is a subset. In set theory, a set is always a subset of itself. But it is not a proper subset of itself because of course it is equal to itself.

As an example, take the set A = {1, 2, 3, 4}. Then B = {1, 2, 3, 4} is a subset of A, but not a proper subset. On the other hand, C = {2, 3, 4} is both a subset and a proper subset of A.