A Relational DataBase Management System or RDBMS is a software application or system for managing relational databases. A RDBMS lets a user (or application) to interact with a database by issuing commands using a conventional or standardized syntax.
Examples of RDBMS includes SQLite, Microsoft SQL Server, PostgreSQL and MySQL.
Every RDBMS varies in complexity and robustest, however they all have the same underlying language in common: Structured Query Language (SQL).
NoSQL
Non-relational data storage and retrieval models exist, such as MongoDB, which uses a document-oriented data storage model.