Before the final release of Android M, there will be two Developer Preview updates6
The initial version of the Android M Developer Preview is now available for downloading, along with the SDK, from the Android Developer web page. Those who flash it on their phone will receive future updates OTA. But as we told you yesterday, be forewarned. You need to back up all of the content on your phone first and if you are not familiar with the process, it is best to leave well enough alone.
The Developer Preview, as you can tell from the title, is not a finished product, so keep that in mind. And the devices that run the preview are limited to the Nexus 5 and Nexus 6 handsets, and the Nexus 9 tablet. Nexus Player will also support the Developer Preview for those with Android TV. And all of these devices have factory images available in case you want to return to Lollipop.
South of this paragraph you can find the entire 39 minute "What's new in Android?" session. You can relive the excitement by clicking on the video.
source: GoogleDevelopers via AndroidCentral