Graph databases are a really neat thing that liberate you from the need to figure out your database schema at the outset and also allow much faster searching than traditional table-based queries across huge datasets. They're ideal for sparse data or for collections of data whose structure/relationships you're not sure about, and also allow very fast searches because the number of steps between different nodes typically grows more slowly than the number of records between different table entries.

There are a bunch of them on the market, Neo4j is probably the most popular (and has lots of good quality introductory text on the website and on youtube). Graph databases are key to many major internet services, eg Google, Twitter, and Facebook are all just really big graph databases.

This particular graph database stores all its data in a single file trading speed and simplicity off against flexibility. I'm not an expert but it seems like it would work very well for search queries, but poorly for tasks involving a lot of contributors like a chat server.