http://ezinearticles.com/?What-Is-SQL-and-Why-It-Is-Important?&id=6909544

A RDBMS is a database management system (DBMS) that is based on the relational model as introduced by Dr. Eager F. Codd. Strictly speaking it should also satisfy Codd's 12 rules ,but in practice there is no DBMS that satisfies all these rules. In fact . most successful DBMS that are considered to be relational violate the relational model in several important ways, including the Structured Query Language . However most database practitioners and researches use the term in a loose way such that most database that support SQL are included. So SQL is a language that is used in RDBMS .......

SQL is one type of database. If we want to save our application data/transaction we need one database. We can use any databse to store the data respective according technolgies which we have used.