RDBMS stands for Relational Database Management System. RDBMS data is structured in database tables, fields and records. Each RDBMS table consists of database table rows. Each database table row consists of one or more database table fields.
An important feature of relational systems is that a single database can be spread across several tables. This differs from flat-file databases, in which each database is self-contained in a single table.
Edgar Codd introduced the relational database model. Many modern DBMS do not conform to the Codd’s definition of a RDBMS, but nonetheless they are still considered to be RDBMS.
The most popular RDBMS are MS SQL Server, DB2, Oracle and MySQL.