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According to an article by Zach Miners over at PCworld:
HealthKit is a new tool for developers in iOS 8 designed to let their apps talk to Apple’s native health apps. HealthKit is meant to pull in information from other apps and devices, like calories burned or heart rate, and make it more useful. For instance, it could allow a nutrition app, with the user’s permission, to tell other fitness apps how many calories the person consumes in a day, Apple says.