TVML for layout and TVJS for logic. TVML is just an XML schema for Apple TV UIs and TVJS a Javascript framework to present the XML templates loaded with your data.

If you're wanting to stream video you can build a full app with almost no Obj-C/Swift (there's still a small shell, though probably nothing custom needs to be here).

The XML templates are a little odd in that they're JS files (e.g. Index.xml.js) and contain the XML as the return value of a global function called Template.

It seems really straight forward though.