The code name for the first version of MacOS is Sierra, and it's available starting today as a free update to current Mac owners (for all Macs from 2010 or later, and a handful of 2009 systems), and will come preinstalled on Mac systems going forward.
MacOS Sierra takes that a bit further, adding new features that are inspired by iOS or are designed to help Macs work better with iOS products, adding further incentive to keep your hardware inside Apple's walled garden, which includes the iPhone, Apple Watch and Mac computers.
Voice assistants are hot right now, from Alexa on the Amazon Echo to Cortana on Windows to the original, Apple's Siri.
It's still the same familiar Siri voice (depending on your region), and she does most of the same things as the iOS version.
A handful of new optimized storage options can keep more files on the cloud, rather than on your physical hard drive.