DBMS : A database management system is a collection of interrelated data and a set of programs to access those data. The primary goal of a DBMS is to provide a way to store and retrieve database information that is both convenient and efficient.
Databases change over time as information is inserted and deleted. The collection of information stored in the database at a particular moment is called an Instance.
The overall design of the database is called the database schema. Database systems have several schemas, partitioned according to the levels of abstraction. The physical schema describes the database design at the physical level, while the logical schema describes the database design at the logical level. A database may also have several schemas at the view level, sometimes called subschemas, that describe different views of the database.
Programmers constructs applications by using the application schema. The physical schema is hidden beneath the logical schema, and can usually be changed easily without affecting application programs.