What is the key difference between a DBMS and RDBMS and any relation between them?
After a while I realised that the original answer has some misleading and wrong information. The content of that answer was taken from Udemy site, but unfortunately that link is broken now. I am posting an entirely new answer for future readers. I removed that answer now.
You can think DBMS as a superset of RDBMS (a special type of DBMS).
A non-relational DBMS (sometimes just called DBMS) stores data as tables (without any relations between tables) or flat files. Document databases and graph databases are examples of DBMS.
On the other hand a RDBMS application store data in tabular form with relations between the tables. An example of RDBMS application is MySQL.