Neo4j is a graph database, a fully transactional database that stores data structured as graphs. A graph is a flexible data structure that allows for a more agile and rapid style of development. You can think of Neo4j as a high-performance graph engine with all the features of a mature and robust database. The programmer works with an object-oriented, flexible network structure rather than with strict and static tables, yet enjoys all the benefits of a fully transactional, enterprise-strength database. The community edition is GPLv3 licensed, while the advanced and enterprise editions are AGPLv3 licensed.