Neo4j a graph database

Neo4j is a graph database. It is an embedded, disk-based, fully transactional Java persistence engine that stores data structured in graphs rather than in tables. A graph (mathematical lingo for a network) is a flexible data structure that allows a more agile and rapid style of development.

According Emil Eifrem the neo4j database outperforms relational backends with >1000x for many increasingly important use cases.