The SharePoint Security Model

SharePoint "Users" (also known as Security Principals) may be either Active Directory users or Active Directory groups.  These "Users" are contained in SharePoint "Groups."  SharePoint Groups are mapped to a permission level for a "Securable Object."  The mapping of groups to permission levels occurs at various "Scopes".  Although any site, subsite, list, library, folder or item is a securable object, not every object creates a scope.