In the contrary to the Facebook example, if Alice follows Bob on Twitter, that doesn’t require Bob to follow Alice back. So a “follow” relation must have a direction indicator, showing which vertex (user) has a directed edge (follows) to the other vertex.
Graphs can be directed and undirected, and that’s one of the interesting properties of graphs. You must’ve seen a popular Facebook vs Twitter example for directed and undirected graphs. A Facebook friendship relation may be easily represented as an undirected graph, because if Alice is a friend with Bob, then Bob must be a friend with Alice, too. There is no direction, both are friends with each other.
In his endeavor Euler showed that the possibility of a walk through graph (city) traversing each edge (bridge) one and only one time is strictly dependent on the degrees of vertices (lands). The path consisting of such edges called [in his honor] an Euler path. The length of an Euler path is the number of edges. Get ready for strict language.