Last concept to revisit, and we'll move on to implementation!
Undirected Graphs
Undirected graphs have edges that do not have a direction. With undirected graphs, we can represent two-way relationships so an edge can be traversed in both directions.
In theory, an undirected graph could be used at Facebook, since both users have to friend each other to build a friendship.
Directed Graphs
Directed graphs have edges with direction. With directed graphs, we can represent one-way relations so an edge can be traversed in a single direction.
A directed graph would be used at Twitter, since the relationship can be one-sided, and you don't need to follow each of your followers.