Android 5.1 vs Android 6: Volume Controls
The volume controls have changed again in Android 6.0 Marshmallow, but this time for the better. Tapping volume up or down will change the volume, but in M a dropdown button appears that instantly lets you access the main three volume controls. Those being notifications, system volume, and alarms. This is how volume controls should always be.
In Android 5.0 all you had was the volume being changed based on if you're inside an app or on your homescreen, and the do not disturb options below the slider. It was an awful change no one liked, and made things more difficult, and with M Google's found a perfect middle ground.