Global.asax and Web.config

This help is very specific and only applies to Microsoft's .NET framework and how to arrange these 2 specific files. It does not go into available options as there is ample documentation at Microsoft or in NET documentation. It is more of a conceptual architecture that is important to understand.

These two files are critical in your website setup and are usually overlooked. Think of them as "Global" application settings. Web.config(WC) is a new powerful configuration file written in XML that enhances old global.asa(GA) files while global.asax(GX) files are basically the same as GA and is needed to separate ASP and NET applications. 

It is important to understand a few things:

You will find that many things that were previously done in the GA file will now migrate to WC file. These include many application/session settings, security, localization and lot's of other stuff. Since .NET handles security much better than ASP you probably will want to integrate/replace any current security. 

If you intend to have several independent applications on the same server....

If you intend to have one "Global" application on the server...

These are core files. How you configure them will greatly affect the usability of your website. Take time to think about how you want to configure these. There are many ways to architect your site, choosing the best will just make life easier in the long run. View lot's of example WC files to see various options. I have yet to see one with every built in option defined, plus you can add your own configuration options, sections, whatever. Your knowledge of GA files is portable to GX files with a few minor adjustments. The real learning curve is the WC files and how best to use them. Understanding of XML is required for WC files.

One way or another you are being dragged into the XML clan. You can come kicking or accept it. Either way XML is inevitable. IBM, Oracle, and Microsoft say so.