QNAP Web UI - How to get the windows in a web app?

I am learning to write web apps. I am currently working on a project and recently purchased a QNAP Nas unit. The web UI for the admin settings is perfect for what I would like, but I don't know how to do the windows thing. Basically you have a menu on the left of the web UI and when you select an admin option it opens a window that you can move around and minimize etc within the app. Does anyone know what technology this is? I don't mind RTFM but I jsut need to know what to read. I have googled and cannot find anything on whatever it is, probably because I don't know what to google for.

Something like this:

If anyone can point me in the right direction of what I can google or read up on that would be great.