SharePoint Designer provides you with three views of a page: Design, Code, and Split.
You should not use SharePoint Designer to edit pages on a SharePoint Web server file system. You should always open a SharePoint site and edit pages from the site over HTTP.
A non-publishing page is the combination of two ASP.NET pages: a master page and a content page.
The Design view of a content page shows the merged view of the master page and the content page, whereas the Code view shows only the code of the content page.
A SharePoint Server publishing page is a combination of three ASP.NET pages: the master page, a page layout, and a content page. You cannot modify a publishing content page by using SharePoint Designer; you must use the browser. ...