This is a network definition file created by the graph analysis program Pajek.
An example file is represented below (italics are comments and are not part of the file format):
*Vertices 3 (number of nodes) 1 "Doc1" 2 "Doc2" 3 "Doc3" *Edges 1 3 4 (vertex_ID_1 vertex_ID_2 edge_weight) 2 3 .5