The key difference is that RDBMS (relational database management system) applications store data in a tabular form, while DBMS applications store data as files. … In an RDBMS, the tables will have an identifier called the primary key. Data values will be stored in the form of tables.
You can say that an RDBMS is an extension of a DBMS, even if there are many differences between the two. Most software products in the market today are both DBMS and RDBMS compliant. Essentially, they can maintain databases in a (relational) tabular form as well as a file form, or both. This means that today an RDBMS application is a DBMS application and vice versa. However, there are still major differences between a relational database system for storing data and a plain database system.