Subpage like this

I observe some links as
www.domain.com/subfolder/
i.e,subpage is saved as INDEX.HTML,am i right?

 

it's can be folder with a index file,
or it can be mod rewrite or isapi rewrite(depends the server)
which it's a change of the url from dynamic(with parameters) to static(without parameters) for seo.

 

most likely it is index.php or index.html . if it is .asp page, it can be default.asp too.

 

It all depends on how the web server software is configured. 'index' is the popular convention, but you could just as easily make the default file 'spam.qsp'.

 

'index' is mostly used by default, so that the server know this is the first file which has to be loaded from the directory