An Edge is a sextuple with, as you may have guessed, six fields. Edges are used to relate nodes to each other. Edges are extensible, and commonly actually have two extra fields (digital signatures, and a graph of properties for the nodes on the edge). The six standard fields are necessary for producing patterns in the graph so that people can extract subgraphs. Signatures and properties fields are not queried for.
standard graph fields
A starting point. Edges are often discovered through their subjects.
A description of the relationship. What is the purpose of the edge? Predicates are commonly known to the client ahead of time.
Something the client may not have already encountered.
Who created the edge?
What knowledge precludes the usefulness of this edge?
How will the client continue to use this edge? What will they query for next?