Basic of Page Authoring in SharePoint

It's just  excerpt from my favorite book Building the SharePoint User Experience :

SharePoint is nothing but a relatively standard ASP.NET solution. So, we should be able to
create custom pages as much as we like.
We need to consider a few things, however. First, SharePoint uses a common _layouts
virtual directory that is mapped to the [12]\TEMPLATE\LAYOUTS folder. This virtual directory is
shared between all sites in the entire farm, so it is a very useful place to put pages that are
going to be used in all or most sites.
If you want your page to be accessible only to a certain site or site collection, it makes
sense to package the pages as a module and make that module part of a feature. That way, you
can deploy or retract the page or pages on a site-by-site basis.