Graph
From Freebase
Freebase is a graph database. This means that instead of using tables and keys found in conventional database to define data structures, Freebase defines its data structure as a set of nodes and a set of links that establish relationships between the nodes. Because its data structure is non-hierarchical, Freebase can model much more complex relationships between individual elements than a conventional database, and is open for users to enter new objects and relationships into the underlying graph.
See also: Graphd