Similar to a family that may have several children, two vertices can also have multiple edges.
In fact one vertex can have multiple loops. The graph below has two loop and multiple edges. It is quite complicated, isn't it?
There is a family of graph that has no loop and has no multiple edges. How do we suppose to call this family? Let's call it simple graph . So next time when you see a graph and you notice it has no loop and it has multiple edges, then you can shout to call it: " hey, you are a simple graph!"
Teknomo, Kardi (2015) Pictorial Introduction to Graph Theory. http:\\people.revoledu.com\kardi\tutorial\GraphTheory\