Union of two sets

The union of two sets is everything in both sets. (with the duplicates removed)
For example if you have the set {3, 4, 5} and the set {5, 6, 7}, then the union of these two sets is {3, 4, 5, 6, 7}. The symbol for union is a capital U. So mathematically, the above sentence would read:

{3, 4, 5} U {5, 6, 7} = {3, 4, 5, 6, 7}
Please note that even though there were 2 fives that the union only contained one.

You can think of the two sets as circles
and the union of the two sets as all the
elements of both sets, like this:

===>

3 4

6 7

<== This kind of a figure, where you show the relationship of sets as circles is called a Venn diagram.