Server and Site Architecture: Object Model Overview (Courtsy MSDN)
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.
Depending on the type of custom application or solution that you are creating, you use different entry points into the object model to obtain the appropriate object from which to start. For example, if you are customizing administration and configuration of a deployment, you can use the static ContentService property to return the current Web service object and its collection of Web applications. To modify settings in the administrative Web application, instead use the AdministrationService property. Collection classes that derive from the Microsoft.SharePoint.Administration.SPPersistedObjectCollection class inherit a GetValue method that you can use to return a specific object from a collection.
If you are creating a Web Part, custom Web service, or Web application to work with site collections, individual sites, or lists, you can use members of the Microsoft.SharePoint.SPContext class to obtain the current site collection, Web site, or list. When you create a Web application in the /_layouts virtual directory, its functionality becomes available to all sites on the Web server. Outside of an HTTP context, such as in a console application or a Windows application, use a constructor of the SPSite class to obtain a specific site collection and to reach various objects within the collection. For more information, see Getting References to Sites, Web Applications, and other Key Objects.
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.
The following diagram shows the Windows SharePoint Services site architecture in relation to the collections and objects of the Microsoft.SharePoint namespace.
Each SPSiteobject, despite its singular name, represents a set of logically related SPWeb objects (see below). Such a set is commonly called a "site collection," but SPSite is not a standard Microsoft .NET collection class, in contrast to SPWebCollection. Rather, it has members that can be used to manage the site collection. The AllWebs property provides access to the SPWebCollection object that represents the collection of all Web sites within the site collection, including the top-level site. The Microsoft.SharePoint.SPSite.OpenWebmethod of the SPSite class returns a specific Web site.
Each site collection includes any number of SPWeb objects, and each object has members that can be used to manage a site, including its template and theme, as well as to access files and folders on the site. The Webs property returns an SPWebCollection object that represents all the subsites of a specified site, and the Lists property returns an SPListCollection object that represents all the lists in the site.
Each SPList object has members that are used to manage the list or access items in the list. The GetItems method can be used to perform queries that return specific items. The Fields property returns an SPFieldCollection object that represents all the fields, or columns, in the list, and the Items property returns an SPListItemCollection object that represents all the items, or rows, in the list.
Each SPField object has members that contain settings for the field.
Each SPListItem object represents a single row in the list.
If you install "Infrastructure Update for Windows SharePoint Services 3.0 (KB951695)," custom solutions may fail if they call the SharePoint object model while impersonation is suspended. If you use Windows authentication and your code calls the SharePoint object model from an IIS worker process, the request must impersonate the calling user’s identity. Windows SharePoint Services configures ASP.NET to impersonate the calling user automatically, but your code may work unexpectedly, or fail, if you suspend impersonation--for example, by calling the RevertToSelf function of the Windows API, or by calling the System.Security.Principal.WindowsIdentity.Impersonate method and passing IntPtr.Zero as the value of the user token parameter. Even if your code does not explicitly revert to self, it might be called by ASP.NET after it reverts to self, such as happens when implementing a virtual path provider; if your code does not impersonate the calling user, it might not function properly.
Next to the Object Model, it is always good to learn some best coding practices when using SharePoint object model. So here you can find that -