For those who are not aware, Google usually updates their Android operating system for smartphones and tablets about once a year (occasionally twice). Usually it is announced at the Google I/O Developers Conference, which is happening on May 28-29 in San Francisco. With the current Android operating system being Android L (also known as Android 5.0 or Lollipop) it is not known precisely what Android M will be, but there are many rumors circulating of its release date, specifics, and update information.
According to Phone Arena, it doesn't look like Android M will be a leap to Android 6.0, but that isn't completely off the table. The current version is 5.1, with updates constantly adding another digit onto that, so Android M is probably going to be a 5 point...something. At the lowest, it will be a leap to 5.2.
As far as any changes go, it doesn't look like Android M will be changing the Material Design principles that have been Android's stock interface. There will probably be minor tweaks, but don't expect anything too revolutionary when comparing the Lollipop homescreen to the Android M homescreen.
There is a possibility of more vocal interaction. That is, the user will be able to interact with Android applications using only their voice, and developers will be able to add hands-free capabilities to their individual software programs. There is also rumor of an Android M feature that is known as Nearby. Apparently, Nearby is supposed to simplify the discovery of and communication with other Android users, similar to Airdrop on iOS.
Android Pit also predicts some improved notifications, so a notification that appears on your smartphone would appear on your tablet or even PC. Most sources agree that Android M should have some increased security features, as most phones come with fingerprint scanners these days and mobile security is becoming a big issue today.
Boy Genius Report (BGR) has stated that there could be a new Photos App that could really improve the user's picture-taking experience. There is also talk that Android M could somehow improve the battery life and RAM efficiency.
One of the biggest question is what sweet treat will Android be named after? It is a minor thing, and in case you aren't aware, Android has been naming their operating systems after sweet treats since version 1.5, aka Cupcake. From there it has gone to Donut (1.6), Éclair (2.0), Froyo (2.2), Gingerbread (2.3), Honeycomb (3.0), Ice Cream Sandwich (4.0), JellyBean (4.1), KitKat (4.4), to Lollipop (5.0.). There are a lot of "M" words that could be sweet treats like Marshmallow, Muffin, or they could name it after a branded candy bar like Milky way or M&M's. That would create some interesting tie-ins.
As for the release date, it should be assumed that Android M will not appear on Android device after the Google I/O conference, because that never seems to happen. Usually, it is a slow rollout, and PC Advisor predicts that Android M will appear on devices at the end of October or early November 2015.