Android Things is a minimal version of Android, intended to power Internet of Things devices (like smart appliances). The first Developer Preview was released at the end of 2016, and new versions have come out steadily since then, most recently with Preview 4.1. Google has now released Developer Preview 5, with plenty of major changes and new features.
Firstly, Android Things is now based on Android O, with apps now able to target API level 26. There are also new Device Management APIs, allowing apps to reboot or reset a given device. Also OpenGL 2.0 is now supported, and platforms with a GPU (like the Raspberry Pi 3) can use hardware acceleration. Speaking of the Raspberry Pi, apps can now configure the Pi's pins as GPIO.
As for hardware support, the NXP SprIoT i.MX6UL development platform now works with Android Things. In addition, Intel Edison and Joule boards will no longer be supported, as Intel is discontinuing them. Developers can still download and install Preview 4.1 on those boards, but no further updates will be made available.
Since this is still a Developer Preview, there are a few known bugs. System power management doesn't work at the moment (so devices can't suspend), Google Play Services requires 2-3 minutes on first boot to setup, and input events from user drivers won't wake the display (if there is one). There are some other device-specific bugs as well, which you can see at the source link.
Still, if you want to try it out for yourself and you have the supported hardware, you can download the latest version from the Android Things Console.
- Android Things Release Notes