A database instance controls 0 or more databases. A database contains 0 or more database application schemas. A database application schema is the set of database objects that apply to a specific application. These objects are relational in nature, and are related to each other, within a database to serve a specific functionality.
For example payroll, purchasing, calibration, trigger, etc. A database application schema not a database. Usually several schemas coexist in a database. A database application is the code base to manipulate and retrieve the data stored in the database application schema.
The database schema changes very infrequently. The database state changes every time the database is updated. Schema is also called intension, whereas state is called extension.