macOS is Apple's operating system for the Mac. It has been around quite a long time, but in its current form it really started in 2001 with Mac OS X 10.1. It's built on top of a UNIX core, which makes it very stable, versatile, and great for developers. It's popular with creators too, as a good platform for video editing and photography. More recently, Apple has begun working to bring iOS apps to the Mac, which could change how MacBooks, iMacs, Mac Pros, and Mac Minis work for everybody.