Apple has today launched macOS Sierra. This comes after a whole summer of testing and many betas later. In this version we find some minor UI tweaks and many improvements. Some of the most notable features include Siri, universal clipboard between iOS and macOS, auto-unlock with an Apple Watch, Apple Pay on the web, and more.
macOS Sierra works on most 2010 and later computers, but also supports the 2009 and newer iMac and MacBook. To get the update, you will need to search for it in the Mac App Store. From there you can download the installer and get the latest version. It is a huge file so it will take a long time to download and quite a long time to install.