The Android M is released by google this year in I/O conference as google did it earlier they did not reveals the name of latest android OS i.e Android M 5.2.
Today google Confirmed the name of Android M as Marshmallow it it is supposed to be as 6.0 as you can see in Android SDK image given above.
This time Android M 6.0 is Coming with lots of new features and the main moto of Android is Totally focused of Android user security (while giving users more granular control over what apps can access),Now it Supports USB type C,Finger print support,Android Pay on mobile and lots of new Feature and Smooth user experience as we know that android M is The Successor OF android L lollipop 5.1.1 with lots of improvement.
See also:Download Final Preview Image of Android M 6.0 for Nexus 5,6,9,Player
The first mobile device/Smartphone which is going to get latest Android M 6.0 Marshmallow will the new Leaked Nexus 5 probably by L.G and one from Huawei.