You have probably read about it and, in fact, your Mac may have actually downloaded the new operating system for you (but not installed it, unlike Microsoft’s practice). Note that Apple has changed the naming of operating system for the Mac from OS X to macOS to better align with the other operating system nomenclature (iOS, TVOS, WatchOS, etc).
What features are there? The big one is Siri on your Mac! Also, you can copy something from your iPhone or iPad and paste it onto your Mac. You can login to your Mac with no password if you are wearing an Apple Watch. Also, there are some very, very nice enhancements to the Photos app (Faces, Places and searching by object - ‘show me all of the photos with dogs or mountains or beaches’, etc.).
Of course, I upgraded (not updated, that is a word keeping what you have up-to-speed) on the first day and, honestly, have had a couple of annoying issues. The main issue is that it keeps asking me for my keychain password every time it wakes from sleep. It is not a problem, I just enter it and move on, but it is getting a bit old. In addition, scanning from my printer appears to be broken. Finally, and this is my mistake, one of my older apps does not run on Sierra so I had to upgrade it.
That said, I can only find a couple of people on the web with the same keychain issue, so it is probably something having to do with my Mac. I would recommend you go ahead and upgrade if you want. If you want to play it safe, wait another week or two for 10.12.1 that will likely fix the keychain issue.
How to upgrade:
There are three really important things to do before you upgrade:
- Backup your Mac (this is always #1, by the way - the most important, critical thing you should do with any computing device).
- Check you apps - look through your Applications folder in Finder. Check the apps you use a lot against this list to see if they are compatible with Sierra: http://roaringapps.com
- Quit all of your apps (I use command-Q on each app to properly quit it - not the red dot) and restart your Mac.
If you have not already received the download of Sierra, go to the Apple menu > App Store and click on the Updates tab. If Sierra is already downloaded but you have not yet installed it you can find it in Finder > Applications > Install macOS Sierra.