Android Marshmallow was released in October 2015. The main new features were:

• Permissions now have to be enabled one-at-a-time as the app requests, a la iOS
• New Fingerprint API
• Visual voicemail built into the Phone app
• Rotating home screen on all devices
• Vertically-scrolling app drawer
• Doze mode, a better way of preserving battery
• Customisable quick settings
• Now on Tap: Contextually available Google Now within apps.
• Three separate audio sliders for audio, media, and alarms.
• USB Type C support

As with other version-specific tags, use this tag if your question only applies to this version of Android, or if it applies to one of the features introduced in this version (i.e. it's specific to "Marshmallow or newer" phones).

Related links

• the official marketing page introducing Marshmallow
• Android Marshmallow on Wikipedia
• 7.0-nougat