Definition :- Schema indicate on overall structure of data item include their record types stored in the database. The first thing you must do to setup a data is to define its structure namely the schema definition.
subschema refers to the application program view to the data item and record type which user would use. Subschema can be derived from on schema. A subschema also referred to as logical view.
Instance is a collection of information stored in the database at a particular amount is called an instance of the database. The database instance is a state of operational database, with data at any given time. The database instance contains a snapshot of the database.