What is the difference between OS X and Darwin OS?

Question

Operating Systems

From OS X 10.0 to the current version, Apple's OS also includes Darwin OS, but how can they work together and how are they different?

Answer 1

Darwin does not include many elements of Mac OS X, such as the Carbon and Cocoa APIs or the Quartz compositor and Aqua user interface, so it cannot run Mac apps.
https://ru.wikipedia.org/wiki/Darwin