A graph database is a databases whose specific purpose is the storage graph-oriented data structures.
Thus it’s all about storing data as vertices and edges. By definition, a graph database is any storage solution where connected elements are linked together without using an index. The neighbours of an entity are accessible by referencing a physical pointer. There are several types of graphs that can be stored: from a “single-type” undirected graph to an hypergraph, including of course property graphs.