In my last article, I started to look at multitenant Web applications. These are applications that run a single time, but that can be retrieved via a variety of hostnames. As I explained in that article, even a simple application can be made multitenant by having it check the hostname used to connect to the HTTP server, and then by displaying a different set of content based on that.
For a simple set of sites, that technique can work well. But if you are working on a multitenant system, you more likely will need a more sophisticated set of techniques.