What is the difference between a directed and undirected graph

Question

What is the difference between these fundamental types?

In drawings I see that the directed has arrows, but what exactly is meant by these arrows in the directed graph a

Answer 1

The difference is the same as between one directional and bidirectional streets - in directed graph, the direction matters and you can't use the edge in the other direction. An undirected graph can be simulated using a directed graph by using pairs of edges in both directions.