You remember ghosting/unghosting in Sharepoint 2003? Yes, I am talking about that concept where you had one physical file serve/act as many, until of course you wanted to customize it.
Sharepoint 2007, built in ASP.NET 2.0, doesn't do that. That is a good thing, because all your customization doesn't get litterred all over the place as multiple files. Your customization sits in the database.
You see, ASP.NET 2.0 has a new concept called Virtual Page Parser. That allows the Sharepoint runtime to query for the aspx as a combination of what is on the file system, and what is in the database. That combination, then appears as a single aspx, and is then run through the pipe of ASP.NET - hence ending up as a class somewhere deep inside temporary asp.net files.
The huge advantage of this approach is of course the ability to revert back to what used to be your page before you completely screwed it up. The hella cool thing is, even Sharepoint designer will clearly show you what pages are customized and what are not. Observe, deeply the exclamation mark below denoting a customized master page.
And before we cause any more confusion, we are not playing with the "Ghosting/Unghosting" terms anymore. We're doing "Customized and Uncustomized" now.