|Two main page types in SharePoint are Application Pages and Content Pages. Application Pages are rendered from the file system and Content Pages are usually rendered from content database. Site Pages are often loaded from content database. Master Pages or Page Templates are actually being loaded from the local file system.|
Pages that are loaded from the file system are known as Ghosted Pages. When pages are being customized using SharePoint 2010 designer the changed page content is stored in the content database, These customized pages are known as Unghosted pages.
Unghosted pages some times gets the content from database and file-system.
Ghosting and Unghosting words are brought from the SharePoint 2003 version, in 2007 these terms are renamed as customized\Customized and SharePoint 2010 these terms are being used as attached and detached to prevent the ambiguity.
However Ghosted and Unghosted terms are still in use and you find in most of SharePoint documentation.
Using SharePoint 2010 designer Administrators and Developers can better manage the Ghosted and Unghosted pages. SharePoint designer loads the page content in safe mode by default. In this mode part of the page content loaded from the file system are high-lighted and can not be edited. You must work in Advanced mode to change this content. Only administrator can edit this content. This Advance mode is to prevent the page being Unghosted.
More about this concept can be found here