Example of isomorphi vs. Different graphs

Below are 3 graphs, which we'll analyze:
Diagram of 3 graphs - download this

Isomorphism

All 3 graphs are isomorphic. Informally, this is because they have the same shape. In fact, "iso" comes from the Greek "isos", which means "equal". "morphic" also comes from Greek, and has to do with "form" or "shape".

Formally, the 3 graphs are isomorphic because there is a mapping of labels that would make them equal. For example, Graph #1 is isomorphic with Graph #3 using the following mapping:
Graph #1 labelGraph #3 label
AB
BA
CC
DD

In words, if you flip A and B in graph #3, you get graph #1.

Difference

Graphs #1 and #3 are obviously different. What might not be so obvious is that graph #1 and graph #2 are the same. Although graphs #1 and #2 may look different, you can see that they're really the same. They're just drawn differently. For example, here's an adjacency matrix representation of both graphs:
Vertexhas edges to these vertices
AB,C
B(nothing)
C(nothing)
DB,C

So, basically, A and D both still go to B and C, even if we choose to draw B and C in different places.

Graph #3 is different from graphs #1 and #2, since the labels don't arrow each other in the same way. Here's the adjacency matrix of graph #3:
Vertexhas edges to these vertices
A(nothing)
BA,C
C(nothing)
DA,C