Android 6.0 Marshmallow is the latest iteration of Android from Google and brings to plate a lot of under the hood changes. One such feature which has already existed in some capacity in Android systems and have been revamped in the latest system update from Google is Automatic Backup. With the new system, Google has changed what data gets uploaded to the cloud for restoration and what doesn't. This has been a part of the early developer time for a long time, but the mechanism of taking backups has changed since the developer preview, employing new rules about data backup.
Up until now, Android could always make backups and synchronize it with Google cloud for the convenience of its users. This back-up was initially handled by Google's Sync infrastructure, but only basic stuff like Gmail, Contacts, Hangouts, Google Calendar, and Drive documents were included in the backup. One of the major drawbacks is that Google Sync only backs up core app data, and never preferences and notification setting, making it a half-baked feature. Another preference Google could never back up would be Home Screen settings, but now, with the Google Now launcher, all data is safely backed up. Google Play nowadays would also reinstall all the missing apps from the Play Store. Though a very arduous process, this is getting increasingly popular among people who need to reset their device. Games have a better option of saving the games progress through Google Play Games, making it much more feasible to a lot of yours. Android 2.2 also had a backup system similar to this, and they have only been improving since the last iterations. It is also really complicated to the regular developers and had a cap of lower than 1MB for files.
In Marshmallow, the system decides what to backup, which is basically all the files in the data folder, and it works for most apps who fulfil some conditions and are opting into the backup feature. From this update, developers have a certain way of controlling whether their apps should be updated. Side loaded apps also have their data backed-up that is indeed surprising. One of the primary requirements for backing up data would be targeting API 23. This is different from the minimum API version as it is only a placeholder for new releases to be supported. Unfortunately, according to reports, there are very few apps that have implemented API 23.