Complex sentence: It was based on the Mach kernel and BSD, an implementation of Unix dating back to the 1970s, and had an object-oriented API based on Objective-C, which in the Mac world is today known as Cocoa.
Split into simpler sentences: It was based on the Mach kernel and BSD, an implementation of Unix dating back to the 1970s. More remarkably, it featured an object-oriented programming framework based on Objective-C. This environment is known in the Mac world today known as Cocoa.