NoSQL databases are characterized by their ability to store data without first requiring one to define a database schema. In Couchbase Server, you can store data as key-value pairs or JSON documents. Data does not need to confirm to a rigid, pre-defined schema from the perspective of the database management system. Due to this schema-less nature, Couchbase Server supports a scale out approach to growth, increasing data and I/O capacity by adding more servers to a cluster; and without any change to application software. In contrast, relational database management systems scale up by adding more capacity including CPU, memory and disk to accommodate growth.
Relational databases store information in relations which must be defined, or modified, before data can be stored. A relation is simply a table of rows, where each row in a given relation has a fixed set of columns. These columns are consistent across each row in a relation. Tables can be further connected through cross-table references. One table, could hold rows of all individual citizens residing in a town. Another table, could have rows consisting of parent, child and relationship fields. The first two fields could be references to rows in the citizens table while the third field describes the parental relationship between the persons in the first two fields such as father or mother.