It's the kernel and basic elements. When Steve Jobs introduced Mac OS X, he stated even then that its kernel would be open source. Watch Steve Jobs state this

It's the kernel and basic elements. When Steve Jobs introduced Mac OS X, he stated even then that its kernel would be open source. Watch Steve Jobs state this here: http://www.youtube.com/watch?v=Ko4V3G4NqII http://www.youtube.com/watch?v=Ko4V3G4NqII It's still great that Apple does this, nonetheless. More open than Microsoft. (Similar source is available for iOS and some other properties too.)