Google Now is one of the best features of Android, and keeps getting better and better. It's an assistant of sorts that can do all types of things. From voice commands, settings reminders or alarms, sending messages or making calls with your voice, to even searching your Gmail and sharing incoming flight information or track packages all automatically for you. Google Now can even do screenshots in Android 6.1 Marshmallow.
However, with Android M you'll still have all of that, but a new feature called Google Now on Tap has been integrated. Long press the home button and instead of opening Google Now, you'll get half the screen being temporarily used by Google Now. This will read whatever is on the screen, analyze it with Google search, and figure out what's on the screen and instantly share information about it. All without leaving whatever app you're already in. Quickly get information about a location you plan to visit, a movie a friend wants to watch, and more. I had a friend say lets go see the new movie "The Martian" and Now on Tap found everything I'd need to know, without ever leaving the email. It will even find apps relevant to what you see on your display.
This has since transformed into an even more powerful feature called the Google Assistant.