A System is a packaged application that can contain sub systems, applications, services and other application elements.
An application is a computer program, a collection of data groups (databases and/or parts of databases) and program modules (components, database management systems, utilities, interactive modules, and system modules), which implements a business function.
A database is a representation of an installed database.
These objects can be created in the following diagrams:
City planning diagram (see City Planning Diagram Basics)
Application architecture diagram (see Application Architecture Diagram Basics)
Service-oriented diagram (see Service Oriented Diagram Basics)