System Landscape 
The portal system landscape is a collection of systems. A system is a portal object comprised of a set of properties that represent an external, or back-end, application. Systems enable a connection to these applications and the retrieval of data from their information stores.
Integration
During portal installation, a Portal Archive (PAR) file containing a portalapp.xml is uploaded and stored in the Portal Content Directory (PCD).
The portalapp.xml is a profile of an external application, providing all the necessary connectivity information to the application. The file can hold the definitions of numerous systems.
You can create a new system directly from the PAR file or from an existing system template. It is also possible to generate a system template based on the PAR file and then use that to create systems.
Some external applications to which you want to connect must have in the portal system a representative connector, the middleware through which the portal content accesses the application.
The portal provides an SAP connector for accessing SAP systems and a Database (JDBC) connector for accessing generic databases. These two connectors are automatically installed to the Web application server during portal installation.
It is possible to see the system landscape of any portal in the System Landscape Overview iView.