SharePoint Services and NameSpaces for Object Model

Identity : Microsoft.SharePoint
Description : Windows SharePoint Services
Path : To add a reference to a Sharepoint API, Right-click the project(in VS) and select Add   Reference. Click the Browse tab and select the following directory:
       C:\program files\common files\microsoft shared\web server extensions\12\isapi

Some useful Microsoft.SharePoint Namespaces follows with Description :

Microsoft.SharePoint
Provides types and members for working with a top-level site and its subsites or lists.

Microsoft.SharePoint.Administration
Provides administrative types and members for managing a Windows SharePoint Services deployment.

Microsoft.SharePoint.Administration.Backup
Provides types and members for performing backup and restore operations on SharePoint sites.

Microsoft.SharePoint.Deployment
Provides types and members for importing and exporting content between Windows SharePoint Services Web sites.

Microsoft.SharePoint.Dsp
Provides the base class for the data retrieval service adapters used in Windows SharePoint Services.

Microsoft.SharePoint.Meetings
Provides types and members that can be used to customize Meeting Workspace sites.

Microsoft.SharePoint.MobileControls
Provides server controls for rendering the mobile forms and view pages used in SharePoint lists.

Microsoft.SharePoint.Navigation
Provides types and members for customizing the navigation structures and site maps of SharePoint Web sites. 

Microsoft.SharePoint.Search
    Microsoft.SharePoint.Search.Internal.Protocols
    Microsoft.SharePoint.Search.Internal.Protocols.SiteData
    Microsoft.SharePoint.Search.Internal.Protocols.SPSCrawl
    Microsoft.SharePoint.Search.Internal.UI
    Microsoft.SharePoint.Search.Internal.WebControls

Microsoft.SharePoint.Security
Provides a set of code access permission and attribute classes designed to protect a specific set of resources and operations, such as access to the Windows SharePoint Services object model, the ability to do unsafe saving on HTTP Gets, and enabling point-to-point Web Part connections.

Microsoft.SharePoint.StsAdmin
Provides an interface for creating custom operations to run at the command prompt in STSADM.EXE

Microsoft.SharePoint.Upgrade
Provides types and members for upgrading a Windows SharePoint Services deployment.

Microsoft.SharePoint.Utilities
Provides utilities for encoding strings and processing user information.

Microsoft.SharePoint.WebControls
Provides server controls that are used on site and list pages in a SharePoint site.

Microsoft.SharePoint.WebPartPages
Provides classes, interfaces, and members for creating custom Web Parts and Web Part pages

Microsoft.SharePoint.Workflow
Provides types and members for associating, initiating, and managing workflow templates and instances.

Microsoft.SharePoint.WorkflowActions
Provides types that represent workflow activities. These activities can be assembled into workflow activities.
  

Some useful Microsoft.SharePoint Classes follows with Description and Example :

SPSite
    Represents a collection of sites in a Web application, including a top-level Web site and all its subsites. Each SPSite object, or site collection, is represented within an SPSiteCollection object that consists of the collection of all site collections in the Web application.
    E.g : SPSite siteCollection = SPControl.GetContextSite(Context);

SPWeb
    Represents a Windows SharePoint Services Web site.
    E.g : SPWeb objWebSite = siteCollection.OpenWeb();    

SPWebCollection
    Represents a collection of SPWeb objects. 
    E.g : SPWebCollection objWebCollection = siteCollection.AllWebs; 
SPSecurity
    Provides static security management properties and methods. 
    E.g : SPSecurity.CodeToRunElevated myCode =
            new SPSecurity.CodeToRunElevated(ShowFeatures);
          SPSecurity.RunWithElevatedPrivileges(myCode); 
SPListCollection
    Represents a collection of SPList objects. 
    E.g : SPListCollection objListCollection = objWebSite.Lists ;
SPList
    Represents a list on a SharePoint Web site.
    E.g : foreach(SPLists objLists in objListCollection)
SPUser
    Represents a user in Microsoft Windows SharePoint Services. 
    E.g : SPUser objUser = objWebSite.CurrentUser ; 
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SharePoint 2010 Object Model

In Sharepoint Object model there are two Important namespaces.

The Microsoft.Office.Server namespace is the root namespace of all Office Server objects and Microsoft.SharePoint is the root namespace for all WSS objects.

The Chart Below illustrates some of the key classes contained in each of these namespaces, as well as to which functional area they belong.

Document Libraries (Microsoft.SharePoint)
SPDocumentLibrary , SPPictureLibrary

Business Data Catalog (Microsoft.Office.Server.ApplicationRegistry.Administration)
EntityCollection , ApplicationRegistry

Features (Microsoft.SharePoint)
SPFeatureDefinition, SPFeatureScope, SPElementDefinition, SPFeature, SPFeatureProperty

Sites (Microsoft.SharePoint)
SPSite, SPSiteAdministration, SPSiteCollection, SPWeb

Meetings (Microsoft.SharePoint.Meetings)
SPMeeting, MtgUtility

User Profiles (Microsoft.Office.Server.UserProfiles)
UserProfile, UserProfileManager

Solutions (Microsoft.SharePoint.Administration)
SPsolution, SPFeatureReceiver, SPSolutionCollection

Lists (Microsoft.SharePoint)
SPList, SPListItem, SPListItemCollection

Notes:
* To use the SharePoint API, your code must reside on one of the machines in a SharePoint
application server farm. Your code can still work with other sites in the farm from any
other site in the farm, but you cannot, for example, work with the SharePoint API from a
machine on which MOSS or WSS is not installed.

* The only practical way to consume SharePoint data and functionality from a remote client is to use the SharePoint web services.

* The object model is not designed to support Remoting