Windows SharePoint Services 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 ;