Directed graphs go in one direction, like water flowing through a bunch of pipes. Undirected graphs don't have a direction, like a mutual friendship. A graph where there is more than one edge between two vertices is called multigraph. One where there is at most one edge is called a simple graph. A simple graph, where every vertex is directly connected to every other is called complete graph. A loop is an edge that connects to its own vertex. Loops are only allowed in multigraphs.