DBMS stands for "Database Management Systems" it includes all Databases.
RDBMS are a special Type of DMBS. "Relational Database Management Systems".
On RDBMS you have Tables. Some Tables have are in relation to other Tables. This is needed to organize complex Data.
Systems like MySQL, MS Access or MS SQLServer are RDBMS.
Other Types are:
ODBMS - Object Database Management System
ORDBMS - Object Relational Database Management System
HDBMS - Hirarchical Database Management System
NDBMS - Network Database Management System