Graph databases are based on graph theory, and employ nodes, edges, and properties. The graph theory is the study of the graphs that are mathematical structures used to model pairwise relations between objects. A graph in this context is made up of nodes, edges which are connected by edges, arcs, or lines.
A graph can be directed or undirected (uni or bi-directional) that might point the direction of the relationship between the edges.
Graph databases can be compared to the Network Model Databases, that were focusing on solving the same problem the interconnected world.
The most popular graph database in the world currently is NEO4J, which is implemented in Java and is using CQL (Cypher Query Language), a language that has definitely inspired the SQL Graph T-SQL language extension.
Niko notes that this is not a fully mature product yet, but it’s an interesting start.