Besides differentiating databases according to function, databases can also be differentiated according to how they model the data.
Data models are abstractions, oftentimes mathematical algorithms and concepts.
You cannot really touch a data model. But nevertheless, they are very useful. The analysis and design of data models has been the cornerstone of the evolution of databases. As models have advanced so has database efficiency. Before the 1980's, the two most commonly used Database Models were the hierarchical and network systems.