Server and Site Architecture: Object Model Overview
Windows SharePoint Services offers a highly structured server-side object model that makes it easy to access objects that represent the various aspects of a SharePoint Web site. From higher-level objects, you can drill down through the object hierarchy to obtain the object that contains the members you need to use in your code.


Server Architecture

The following diagram shows the Windows SharePoint Services server architecture in relation to the collections and objects of the Microsoft.SharePoint.Administration namespace.

• The SPFarm object is the highest object within the Windows SharePoint Services object model hierarchy. The Servers property gets a collection representing all the servers in the deployment, and the Services property gets a collection representing all the services.

• Each SPServer object represents a physical server computer. The ServiceInstances property provides access to the set of individual service instances that run on the individual computer.

• Each SPService object represents a logical service or application installed in the server farm. A service object provides access to server farm-wide settings of the load-balanced service that a respective service instance implements. Derived types of the SPService class include, for example, objects for Windows services, such as the timer service, search, Microsoft SQL Server, the database service, etc. and also objects for Web services, such as Windows SharePoint Services or services in the Microsoft Office system

• An SPWebService object provides access to configuration settings for a specific logical service or application. The WebApplications property gets the collection of Web applications that run the service.

• An SPDatabaseServiceInstance object represents a single instance of a database service running on the server computer. The SPDatabaseServiceInstance class derives from the SPServiceInstance class and thus inherits the Service property, which provides access to the service or application that the instance implements. The Databases property gets the collection of content databases used in the service

• Each SPWebApplication object represents a load-balanced Web application based in Internet Information Services (IIS). The SPWebApplication object provides access to credentials and other server farm wide application settings. The Sites property gets the collection of site collections within the Web application, and the ContentDatabases property collection of content databases used in the Web application. The SPWebApplication class replaces the obsolete SPVirtualServer class; but it can still be helpful to think of a SPWebApplication object as a virtual server; that is, a set of one or more physical servers that appear as a single server to users.

• An SPContentDatabase object inherits from the SPDatabase class and represents a database that contains user data for a SharePoint Web application. The Sites property gets the collection of site collections for which the content database stores data, and the WebApplication property gets the parent Web application.

• An SPSiteCollection object represents the collection of site collections within the Web application. The Item property or indexer gets a specified site collection from the collection, and the Add method creates a site collection within the collection.

Source:MSDN