Before reading this post please read the previous two articles ( ISAPI extensions and ISAPI filters ).
I thought of writing my notes in the form of a bulleted list because this is not a detailed article.

• ASP.NET 2.0 is an ISAPI extension that provides managed alternatives to ISAPI extensions ( HTTPHandler ) and ISAPI filters ( HTTPModule ).
• SharePoint 3.0 is based on ASP.NET 2.0 which is very extensible ( has extension points that support virtual paths and extensions to parsing )
• SPVirtualPath Provider extends ASP.NET’s VirtualPathProvider and checks the content DB to know if a user customized (unghosted) the page.
• SPPageParserFilter extends ASP.NET’s PageParserFilter and is used to prevent the pages from containing undesirable elements ( decided whether the pages can contain code, controls that are allowed, maximum number of controls permited on a page,..)