Difference between application pages and site pages.

Hi all, I am new in SharePoint and want to know about difference between application pages and site pages. further more I have read that:

"application pages are generic pages which will be used by all the sites in a site collection"

what if I deployed it under the sub site's _layout folder rather then deploying it in the root site's _layout folder?

I have also read that:

"Application Pages are pages in WSS that cannot be customized. Example of application page is setting.aspx. Application Pages are more scaleable and yield better performance compared to site pages."


how application pages are scaleable when I cant customize them?

please help to get these confusions clear, your quick response will be highly appreciated.