A digraph contains vertices and edges as above, but the edges are directed. In other words, the edge relation need not be symmetric. The edge v1→v2 does not imply the edge v2→v1. Edges are drawn using arrows, rather than line segments. Again, loops and multiple edges are usually prohibited.
A vertex has degree n if there are n edges connecting that vertex to other vertices. The number of edges is half the sum of the degrees.
In a digraph, vertices have indegrees and outdegrees. The number of directed edges is the sum of the indegrees, or the sum of the outdegrees.