HealthKit is Apple's centralized health and fitness database that stores encrypted user data locally on devices. It allows apps to read and write standard health data types through the HealthKit framework. Developers must configure their apps with the proper HealthKit entitlements and privacy strings to access the data. HealthKit is available on iOS and WatchOS but not iPad. It stores three types of data - characteristic profile data, point-in-time samples, and source metadata. HealthKit provides classes like HKSample to represent different data types like categories, quantities, correlations, and workouts.