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Back-enders are developers who convert business logic into functional systems capable of automating processes. Compared to front-enders their main focus is on making things work, in the broadest sense of the word.
A backend app developer works on iOS (iPhone and iPad) or Android (phone, tablet) apps and makes them functional. They take the interface designed by a frontend app developer (usually these are separate roles, but sometimes a developer can do both) and link it with functionality to make an app screen work. Think of a ball bouncing on a screen when you swipe the screen of your phone.
Apps often integrate with cloud functionalities. These cloud APIs are separate systems that are built by backend web developers. Or with an external piece of hardware, such as a card reader or a label printer.