"A relational database is a database that conforms to the relational model, and refers to a database's data and schema (the database's structure of how that data is arranged). Common usage of the term "Relational database management system" technically refers to the software used to create a relational database, but sometimes mistakenly refers to a relational database."

Source: http://en.wikipedia.org/wiki/Relational_database

Relational database management systems include Microsoft SQL Server and Oracle.