What is the relationship between BGL's edge_descriptor and edge_iterator?

What is the relationship between an "edge_descriptor" and what an edge_iterator dereferences? I'm having problems coding the following: myFunct(GraphT& g) { typedef graph_traits<GraphT>::edge_descriptor Edge; typedef map< Edge, Edge_Datum> Edge_Data; Edge_Data edge_data; graph_traits<GraphT>::edge_iterator ei, ei_end; for(tie(ei,ei_end)=edges(graph);ei!=ei_end;++ei) edge_data.insert( make_pair(*ei, Edge_Data())); << blows up here. }