Let's start by exploring the answers to the question, what exactly is a NoSQL database? NoSQL is a common term for database technologies that deviate from the traditional Relational Database Management System (RDBMS) concepts. The common reason for these database solutions to deviate from RDBMS database standards is to achieve and set a better standard of availability and partitioning capabilities than traditional RDBMS solutions.
To introduce you to this concept, we should have a look at the Brewer's theorem, which is otherwise known as the CAP theorem:
"It is impossible for a distributed computer system to simultaneously provide all three of the following guarantees: Consistency, Availability, and Partition tolerance."