A relational database management system (RDBMS) is a program that lets you create, update, and administer a relational database
The full definition from Whatis.com can be found <a href="http://searchsqlserver.techtarget.com/sDefinition/0,,sid87_gci214260,00.html">here</a>
For multiple articles about relational DBMSs, the relational model, and their relationship (no pun intended) to Oracle, check out this Learning Guide:
http://searchoracle.techtarget.com/generic/0,295582,sid41_gci1087688,00.html
--Tim