Assembly: Microsoft.SharePoint (in Microsoft.SharePoint.dll)
Available in Sandboxed Solutions: Yes
Available in SharePoint Online
If the SPWebTemplate represents a site definition configuration, this property corresponds to the ID attribute of a Template element in a WebTemp*.xml file. (See Understanding WebTemp*.xml Files for more information.)
If the SPWebTemplate represents a Web template, this property corresponds to the BaseTemplateID attribute in the WebTemplate element in a elements.xml file contained in the .wsp file that stores the Web template. The value is the ID of the original site definition configuration which was the foundation of the site which served as the source of the Web template.