Here’s a link to a brochure describing the ranking process.
Interesting to view the list with Brexit in mind. As is, 3 of the top 10 (As noted in the comments, Switzerland is not part of the EU), including #1 Oxford, are in the EU. After Brexit, 0.
International university rankings 2016-2017
Note that Switzerland is not in the European Union.
Fixed. Makes the point that more dramatic, I think. Thanks for the edit.