WSS v3 Core Managed Assemblies

Microsoft.SharePoint.dll
–Core objects for top-level sites, subsites and lists
–Core objects for administrative functions

Microsoft.SharePoint.Security.dll
–Provides CAS permission and attribute classes that can be used for declarative security programming

Microsoft.SharePoint.Search.dll
–Classes for administration of search and leveraging search

Microsoft.SharePoint.ApplicationPages.dll
–Types and members that directly support WSS pages, not intended for use by your code.

Microsoft.SharePoint.WorkflowActions.dll
–Contains many custom activities created to leverage Windows Workflow Foundation

stsSoap.dll
–Contains the classes the implement the WSS Web services –Meant for use by client applications leveraging Web services