“App” used to simply be short for Application. The systems or programs we run our computers. Gradually in the course of the late nineties the Application became the Solution and the new applications designed to run natively on mobile phones like iPhones and Androids became nicknamed “Apps”.
When the battle between desktop and smartphone began to heat up Microsoft launched windows 8 in a move to capture the Mobile world with its new OS and new phone. The Windows interface became phone-like and the solution/system/application now became suddenly became an “App” too, or Windows App.
The upshot is that at time of writing 2017, you have to work it out for yourself each time someone mentions an app because they could be stuck in any of thee time-warps
The “Explained series” is planned to build into a trustworthy collection of explanations and commentaries that can be trusted to tell the story straight without any bias and attempt to make the subjects accessible to the layman. The latter is not always easy as some of these terms refer to genuinely complex subject matter, while others are simply too vague to pin down (there’s another word for that).
If you want an answer on something and you can’t find it easily, please use the comments section to just ask and I will appreciate not having to research the next topic.