This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
I found the definition for RDBS is
" RDBMS (Relational Database Management System) is a type of DBMS (Database Management System) which stores the data in the form of tables. "
So what are the other databases ?
Relational databases are the most common now, you need to go back in history (or forward with Google's BigTable system) to see others. For example, some database systems store things as entities with attributes which could be open. In other words, entities have an undefined list of attributes and some entities may have attributes others do not.