Basically this article will describe that how sharePoint is deciding responsing when it gets the request for the pages.
In SharePoint request can be for either Ghosted pages or Unghosted pages. Ghosted pages are something unCustomized and resides in the file system and UnGhosted pages are something Customized and resides in File system.
How sharepoint decides that from where it has to look for the requested pages. SPVirtualPath Provider helps in it. The below picture is self explanatory.
virtual provider is an abstraction that loads the page from the bottom or the file system depending on whether or common custom pages and so with the ASP.NET runtime.