I'm not op, but I think this would be like xhp or jsx. Really it's any templating language that works closer to the DOM level and can perfom optimizations like understanding which close tags don't need to be output
It wasn't Closure Templates - those are what's used on the Apps side of Google (GMail, Google+, Docs, etc.). The technology in question lies in a grey area as far as confidentiality goes...it's been mentioned once in a recruiting presentation and there are references to it in other Google open-source projects...but I figure I would be safer by not naming it.