Zanzibar is a global highly available distributed permissions system used within Google to power application permissions for things like Maps, YouTube, Calendar, Doc/Drive, etc. They wrote about it in a paper[0] that was widely discussed on HN at the time[1].
The service stores relationships between people, other people, and data, in a giant directed graph. There are primitives for querying and processing that graph to make permissions decisions. The majority of the rest of the engineering effort is spent on replicating the data globally and caching permissions decisions regionally and locally, since permissions don't lend themselves very well to sharding or siloing along service boundaries.
For the 5+ explanation, I wrote a little bit about my digestion of the paper and what the important parts are here[2].
