Sierra, also known as macOS 10.12, is the 13th version of OS X, released on September 20, 2016. It followed El Capitan (OS X 10.11) and precedes macOS High Sierra. Sierra was the first version of OS X that Apple officially labeled "macOS" instead of OS X.
Like other incremental releases of Apple's desktop operating system, Sierra built on previous versions, incorporating the same user interface and adding several new features. Notably, Sierra was designed to work more seamlessly with other Apple devices, such as iPhones and Apple Watches. For example, Sierra was the first version of macOS to support Universal Clipboard, which allows you to copy something on your phone and paste it on your Mac, or vice versa. It also supports automatic login using an Apple Watch. Safari, the web browser included with macOS Sierra, supports Apple Pay using an iPhone or Apple Watch.
Sierra also increased iCloud integration by syncing the Desktop and Documents folders with iCloud drive. This makes it easier to keep files consistent across multiple Macs. It was also the first version of Apple's desktop OS to include Siri, Apple's voice assistant that was introduced in iOS. Siri can be accessed by clicking the icon in the right side of the menu bar or by pressing and holding Command+Space Bar. You can speak commands, such as "Show my Downloads folder" or "Make the screen brighter," instead of performing the actions with your keyboard and mouse.
NOTE: Before Mac OS X (introduced in 2001), Apple's desktop operating system was called "Mac OS." With the release of Mountain Lion (OS X 10.8) in 2012, Apple changed the name from Mac OS X to just "OS X." The switch to "macOS" with Sierra marks a return to the "Mac OS" name, while using the same naming convention as Apple's other operating systems – iOS, watchOS, and tvOS.