Notation problem in Sets

Question

Consider two sets : $\{1,2,3\}$ and $\{1,2,3,4\}$

What is $\{1,2,3\}\setminus\{1,2,3,4\}$?

I am giving an example of the two sets. I want to understand what that $\setminus$ of the two sets above imply?

Answer 1

$A\setminus B$ is the set containing all elements of $A$ not in $B$. It is a subset of $A$.

Here, as $A\subseteq B$, $A\setminus B=\varnothing$.