An operation in set theory is a function of two sets, that returns a set.
Common set operations include Union ($\cup$), Intersection ($\cap$), Relative complement ($\setminus$), and Cartesian product ($\times$).
The union of two sets is the set of elements which are in one or the other, or both
The intersection of two sets is the set of elements they have in common
An unordered collection of distinct objects.