Every year, Apple introduces a new version of OS X, but this year the new version got a completely new name. If you're upgrading to the latest version of Apple's operating system, you'll be upgrading to macOS, not OS X. The underlying architecture and overall experience remain basically the same, but the name is now consistent with Apple's iOS and watchOS. But macOS Sierra is about a lot more than just the name. Here's what you need to know.
1. macOS Sierra is available as a free upgrade right now, but many people got early access to it by taking advantage of a public beta Apple made available a few months ago. (Registered developers and PCMag had access to a very early beta version before that, and we've been testing it ever since.) If you want early access to the next version of macOS (or the iPhone's iOS) you should sign up at beta.apple.com. Of course, you use a beta OS at your own risk. While Apple's betas tend to be reasonably solid by the time they are widely released, we still don't recommend installing them on mission-critical systems.
2. MacOS Sierra is a free upgrade and works on any Mac dating back to 2010, and on MacBooks and iMacs from 2009. Older hardware will still work with OS X 10.11 El Capitan, which you're probably using now. Some new features may not work on older Macs that can run the OS, however.
3. Siri finally comes to the Mac. Just as Microsoft built its speech-driven assistant Cortana into Windows 10, so Apple built iOS's Siri into macOS. Sierra's Siri won't respond to "Hey, Siri" because your Mac doesn't listen to you all the time, but she pops up with a keystroke (the default is Fn-space) or a mouse click and finds local restaurants or pictures you took last week, or tells you weather, sports scores, and much more. You can drag Siri's answers from a pop-up window to the notification pane or drag images into documents. A new preference pane lets you decide whether Siri will talk back to you or simply type out her answers on screen. You can also choose her gender and nationality.
4. Sierra, in combination with iOS 10, introduces a universal clipboard that lets you save text on a Mac and paste it into your iPad or iPhone, or the reverse. Other ways the Apple ecosystem gets more tightly integrated include automatic unlocking of a Mac when an unlocked Apple Watch is close by (and on your wrist).
5. Picture in Picture comes to the Mac, too. As on the iPad, you can extract a video from a Web page and have it play on your macOS desktop without the surrounding Web page. The video stays playing, in the same position, even when you switch among multiple desktops.
6. The Finder gets some welcome additions—an option that automatically empties items from the trash after 30 days, and an option to put folders first when the Finder lists files and folders by name. Windows has had similar features for years, and they're overdue on the Mac.
7. Automatic uploading to iCloud from your Desktop and Documents folders. Unless you turn this feature off, every file and subfolder in your Desktop and Documents folder automatically gets uploaded to your iCloud drive—a feature that may prompt you to buy more storage than the 5GB you get free. The point of this is that all the files you use most often will be instantly available on any Apple device or Windows machine with iCloud installed. Sierra will also advise you on which files you might want to remove from your Mac and store entirely in the cloud, downloading them when needed.
8. The Memories feature in Photos uses facial recognition and other smarts to combine related photos into Ken Burns-style movies that pan and zoom through your pictures, and it can even add a soundtrack. I have mixed feelings about this feature, as I suspect the gee-whiz quality may not have much staying power. Maybe you'll like having your software do your photo curating for you, however.
9. Tabs are everywhere. Maps, Mail, and any other app—whether or not Apple wrote it—that normally opens separate windows can now use multiple tabs instead. I tried this feature in Mail, where I typically have three or four message windows open, and was instantly glad to have the reduced clutter and easier navigation that this feature offers.
10. You may have heard about Apple's new Apple File System (APFS) that brings new security, speed, and reliability features to macOS. This is available to developers now, but won't really be ready for consumers until 2017, which means you won't see your Mac making use of it until the version of macOS that comes after Sierra. Apple plans to take its time testing APFS, the biggest change in its file system in 30 years. For more on the new system, you can read What macOS Sierra's New APFS File System Means to You.