Almost every application use data storage. It can be just a file or something more complicated like a Database Management System.
We use relational DBMS since 1979, they are built on top of relational algebra. But it is so much time past since that times and there were so many changes in technologies. Web has been developing incredibly fast, and unfortunately classic model doesn’t fit very well for new requirements.
This is why NoSQL movement appeared. This name was created by marketing analysts to designate any technology that tries to find a way to bypass a restrictions of classic model.
The most important thing in a domain model is a relationships between objects. And in a Graph model it is a basic concept. That’s why representation of relationships are so native in this NoSQL approach.
To have a brief introduction into graph model and how it implemented in OrientDB, check out this presentation.