Whats the difference between Collector and ArcGIS App?
You can hide your server URL by securing your feature services and setting up a proxy on the service in AGOL. This will hide the URL of your server to the user and it will show up as "utility.arcgis.com/.." and you will not be able to see the actual server URL. You can do this by 1)enabling ArcGIS Server security on your service, 2) In AGOL under My Content click "Add Item" and Add from the web. You can then embed the credentials on the service if you want or have it set to prompt your users of the map.
Russ
I see you are in Russ' hands for the questions on your particular maps. For iOS version, yes, iOS 8 is supported for the Collector and Explorer apps, and both of those apps are regularly updating.
From below-Kylie
Taken straight from the help:
"Collector for ArcGIS focuses on data collection and provides a streamlined experience for working in the field, while the ArcGIS app focuses on general map use, query, and feature editing."
For a bit more info, you can also look at the differences with the ArcGIS app and Explorer, and many of the same differences apply with the ArcGIS app and Collector:
"ArcGIS for iOS and Android were the first native viewers of maps hosted in ArcGIS Online or Portal for ArcGIS. Explorer for ArcGIS is the modern, native experience for working with your organization's maps. As the ArcGIS system evolves, additional functionality will be brought into Explorer, while the ArcGIS for iOS and Android apps won't be updated with new features and functionality."