It's all in the title.
I think the ArcGIS App was the first mobile app for Android and iOS. I believe it is being sunsetted. Collector and Explorer are the mobile apps that replace the ArcGIS App. Collector is the mobile app you choose if you want to edit the data, it cannot consume ArcGIS Online web maps unless there is at least one layer in the map that is editable. Explorer is just for viewing, it can consume all ArcGIS Online web maps. Sure there are more differences that someone in the community can comment on. Hope this is helpful
Thanks for your quick answer, that's all I wanted to know.
Retrieving data ...