Definition - What does Non-Relational Database mean?
A non-relational database is a database that does not incorporate the table/key model that relational database management systems (RDBMS) promote. These kinds of databases require data manipulation techniques and processes designed to provide solutions to big data problems that big companies face. The most popular emerging non-relational database is called NoSQL (Not Only SQL).
Techopedia explains Non-Relational Database
An interesting aspect of a non-relational database such as NoSQL is scalability. NoSQL uses the BASE system (basically available, soft-state, eventually consistent). Non-relational databases forgo the table form of rows and columns relational databases use in favor of specialized frameworks to store data, which can be accessed by special query APIs. Persistence is an important element in these databases. To enable fast throughput of vast amounts of data the best option for performance is "in memory," rather than reading and writing from disks.
Relational databases use the ACID system, which ensures consistency of data in all situations of data management but obviously takes longer to process because of all those relations and its branching nature. However, the BASE system loosened up the requirements on consistency to achieve better availability and partitioning for better scalability.