Let A and B are two set. Then union of A and B is denoted as AUB.

i.e. AUB means elements are either in A or in B are in both.

Example:

Let A denotes numbers less than 5

So A = {1,2,3,4,5}

Let B denote prime numbers less than 10 

So B = {2,3,5,7}

Now AUB = {1,2,3,4,5,7}

The universal set is the set of all elements under cosidiration.

Example: Let U denotes set of all prime numbers

So U  = {2,3,5,7,11,13,17,19,23,..............}