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Schema and Instance: 

The description of the database is known as Schema. Schema is specified during database design and does not change frequently. However, a Schema diagram does not depict the entire picture of a database. The data types of each attribute, the relationship among files, constraints imposed on an attribute, etc. are not depicted by Schemas. Following is an example of some schemas-

Instance has defined a snapshot of a database at a particular moment of time.

When an attribute is added to an existing schema, it is known as Schema Evolution. At the time of schema specification, the database is in the initial/new state where no data is inserted. When data is entered into the database for the first time, it is in the initial state. A schema is also referred to as intension  and database state is known as extension of the schema.