The atchiteture of a database system is the design of the database for a given application. In some cases an application may be divided into multiple databases.
Each database contains one or more tables, each of which contain one or more fields.
Fields within a database contain the data required by the associated application, and the business that uses it.
The architecture of a database is the physical design of the datbase, its tables, and associated objects such as Stored Procedures, Triggers, Functions, and routines.