Android 6.0 Marshmallow is out, it's official, and it's already making its way to Nexus devices through over-the-air updates as well as through manual flashing of the software by those individuals savvy enough to do so. Android Marshmallow is the new and tasty and Lollipop might as well be considered a reheated lunch that no one wants to eat the next day. All jokes aside, there are those that are surely going to see the coming of Marshmallow to Nexus devices and wish they could have their taste. Rest assured, for most of the current devices and last year's flagships the software is likely coming and will just be a waiting game.
As for the software, the latest changelog always gets posted up to the Android Open Source Project at some point which details the changes made in the latest version of Android to hit the airwaves. In this case it's no different and the AOSP changelog is now available for your consumption if you should choose to want to read it, look it over, what have you. Keep in mind that this is just a bunch of commits and thousands of lines of them at that, so there isn't going to be any clear written out description of what's changed in the software sentence after sentence. Having said that, this also isn't going to be a quick read so if you only have a few minutes, you may want to postpone reading up on it.
The changelog was only so far off give that the software is already being pushed out the Nexus smartphones and tablets. If you haven't gotten the update on your personal Nexus device yet, you can check out the OTA downloads list here and grab the file download and install the OTA package manually on your own. You can also flash the factory images which were posted by Google up to the web yesterday. For anyone not on a Nexus device, multiple OEMs have already confirmed the software to be pushed out to various devices including Motorola, HTC, and just as of today Sony has committed to pushing Marshmallow to a number of their devices as well.