This[0] video from Apple's WWDC gives a nice overview of how Differential Privacy is being used in iOS. Basically, Apple can collect and store its users’ data in a format that lets it glean useful info about what people do, say, like and want. But it can't extract anything about a single specific one of those people that might represent a privacy violation. And neither can hackers or intelligence agencies.

[0] https://developer.apple.com/videos/play/wwdc2016/709/?time=8... (the "Transcript" tab has the text of the video if you want to read instead of watch.)