Having a web interface to control a media centre is a very useful tool, and most major media centre software have one, including mythTV and Windows MCE. MediaPortal was unfortunately missing one, so a group of MediaPortal users jumped in and created one. It allos you to view the program guide and set recordings, as well as operate remotely via the on-screen remote.
I'm the developer of the External Control Plugin/web service which the web interface uses to communicate with MediaPortal. Written using C#, it communicates with MediaPortal using .NET remoting, exposing methods to get database data, control MediaPortal, and to get the current MediaPortal status among other things. The .NET remoting is then re-exposed via a web service to allow non-.NET apps to use it, such as the MediaPortal Web Interface, which is written in PHP at the moment.