Apple today released the second iteration of the beta for the next version of its desktop OS, macOS 10.12 Sierra. The seed is available to all registered Apple developers.
The main new feature in macOS 10.12 Sierra is Siri, a feature that iPhone users have enjoyed since the days of iOS 5 and the iPhone 4s. There are other features too - which can also be found in iOS 10 - such as automatically organizing your photos by faces. You can check out our hands-on with the first beta right here.
The release notes for beta 2 don't say anything about new features, but they do provide a long list of fixes and known issues. Keep in mind that Xcode 7.3.1 is not compatible with macOS Sierra, so you'll need to be running the Xcode 8 beta, which also saw a new release today.
After installing the new build, you should also see an update waiting for you for iTunes 12.5, which is a minor release.
The macOS Sierra beta is still only available to developers. If you're waiting on the public beta, it's probably going to be about two weeks, as that usually shows up after Developer Beta 3. If you're eligible, you can check out the release notes right here.