What is the difference between $\{1, 2\}\cup\{3\}$ and $\{1, 2, 3\}$? I have a simple question related to set theory. 
Is there any difference between the following two sets?

What is the difference between $\{1, 2\}\cup\{3\}$ and $\{1, 2, 3\}$?
There is no difference; they are the same set.
The union of $\{1,2\}$ and $\{3\}$ is equal to $\{1,2,3\}$.  The intersect of $\{1,2\}$ and $\{3\}$ would be equal to $0$.