Universal Set :

Universal set is a set which contains all objects, including itself.

eg : the set of all other sets of number.

Union of Sets :

The Union of two sets A and B, written AUB, is the set that consists of all elements belongings either to A or to B (or to both).
A set which contains all sets is called a universal set. It is a super set of each of the given sets. It is the set which contain all objects. It is generally denoted by U.

Let A = {a, b, c}, B = {a, c, e, f}, C = {b, d, f, h} then U = {a, b, c, d, e, f, g, h}.

 The union of some number of sets is the set of all distinct elements in the given set.