Set (Sets) refers to a group of things, whether they are people or animals, which are mathematical expressions can identify members of the group, and called on "members of.
Set (Sets) represents a group of objects, whether a person, animal, object, or mathematical expressions. This can identify group members. And that the group members "Members of the Set
Set (Sets) means the things such as people, animals, objects, or mathematical expressions, which can identify the members in the group. And call the group members that "souvenir.