A non-relational database, also known as a NoSQL database, is a type of database that does not use the tabular schema of rows and columns found in most traditional database systems. Instead, non-relational databases use a storage model that is more flexible and can be optimized for specific requirements of the type of data being stored. Non-relational databases store data in a non-tabular form, which can be based on data structures like documents, graphs, or key-value pairs. They are often used when large quantities of complex and diverse data need to be organized, and they offer rapid scalability to manage large and typically unstructured data sets. Non-relational databases do not require a schema, which means they can store large amounts of data with little structure and provide schema-free or schema-on-read options. They are a natural choice for big data and real-time web applications that require efficient querying and fast results. Some popular NoSQL databases include MongoDB, Apache Cassandra, Redis, Couchbase, and Apache HBase.