Each .aspx page is rendered by one of two possible parsers. When a request is received for an .aspx page, the SharePoint isapi filter determines who will handle the rendering of the page—Asp.net orthe SharePoint SafeMode parser. The first parser, Asp.net, requires the least amount of introduction. The second parser is unique to Windows SharePoint Services.
The intent of this discussion is to cover the differences between the two parsers. To be very clear, this discussion applies to pages which come from the main application root of a SharePoint virtual server. Pages which originate from either the "_layouts" or "_vti_bin" virtual directories can be excluded from the discussion.
As everyone knows, all pages within SharePoint are stored in the database. This effectively means that for each document, you will find a row in the docs table for that document. The actual file is stored in the Content column. This is true for all files. However, there is one exception - some .aspx pages don't actually have their content stored in the database. Instead, these pages reference files which exist on the server's file system. These pages are considered ghosted pages.
From a technical standpoint, ghosted pages are those rows in the docs table which have null values for the Content column and a non-null value for the SetupPath column which points to a file on the file system itself. The referenced file essentially serves as a template and content source.
What pages are ghosted? For example, the default home page is a ghosted page. Any web part pages created the via New Web Part Page user interface also ghosted.
As you can see, I've described ghosted pages as the exception to the rule. What does it mean if a document doesn't reference a template on the file system? Or, more to the point, the Content column actually contains data? These pages are known as unghosted .aspx pages and they are routed through the SafeMode parser.