What is a good approach to start developing web applications? [on hold]

I would like to know what kind of programming language you need to know/learn to develop flexible web applications? I found this one for instance:

http://ift.tt/1MqljDe

As you can see you have serval windows which you can drag&drop, adjust in size and add and delete. It's like a program which runs in your browser. But I cannot tell if that is flash, HTML5, Java or whatever.

So I would like to know what do I have to learn to create such kinds of application?

Thanks for your help!