Most websites are deployed to a single location. Even in a load balanced scenario, we still think of it as a single site. However, it is much rarer to find developers that have developed web sites where the same code is installed to multiple locations for multiple customers. These types of sites are truly web applications and they are a different beast than websites.

Web applications present substantially more complex design decisions that do websites. For example, you can longer “just change a label’s text” and re-deploy. You have to think about the implications of every change against all other clients. Generally, any and all customizations must be controllable via some entity outside the site such as a database or client-specific config file.

Perhaps the most useful skill a developer can learn to this end is to design all their sites to run in a directory as opposed to assuming they are running at the root of the site. This was a chore in classic ASP (one of the many reasons for everyone to stop using it). In ASP.NET, this means that all paths must be prefixed with a tilde (e.g. ~/). In some cases, you must still jump through a couple of hoops to make this work.  In .NET, with web applications, designing your applications to run in a directory gives you the ability to create a client-specific config file above the application config file like so:

/ (site root)
    Web.config  - client specific settings
    /MyApp
        Web.config - application and client-agnostic settings

A key feature in designing web applications is to ensure a "fire-and-forget" type of deployment which makes turn around on your bug fixes substantially quicker. When you post a fix, you want to be able to blow out the application directory and completely replace its contents. That means that no client specific settings can be stored within the application either in code or in its web.config file. Thanks to .NET's ability to "roll up" config files, the "only" requirement on the developer is to ensure they develop their sites with no assumptions about what is above their site's directory.