What are roles?
“Functions” of nodes in the network – Similar to functional roles of species in ecosystems
• Measured by structural behaviors
• Examples – centers of stars – members of cliques – peripheral nodes
Why are roles important?
Roles VS Communities:
• Roles group nodes with similar structural properties
• Communities group nodes that are wellconnected to each other
• Roles and communities are complementary
the distance between two vertices in a graph is the number of edges in a shortest path connecting them.
the eccentricity 
The radius 
The diameter 
原文：https://www.cnblogs.com/dulun/p/11914346.html