Mac OS X 10.0 based on Darwin
Architecture
Mac OS X is built on Darwin, a Unix-like operating system derived from FreeBSD. Darwin includes a new kernel, XNU, derived from Mach and BSD, as a replacement for the Mac OS nanokernel used in classic Mac OS.
Unlike Mac OS 9, Mac OS X has protected memory and preemptive multitasking. This means that if an application's memory becomes corrupted due to a bug, the application will crash without the entire system crashing and needing to be rebooted.
Mac OS X also had support for OpenGL, AppleScript, and the Carbon and Cocoa APIs.