SUP is a separate server running on a windows.
The depends on the requirement. Mainly we can classify the SAP mobile app development as Native apps and Hybrid apps.
If you want to develop native apps means you need the following skills based on the device platform you are targeting.
- IOS - Objective C
- BB & Android - Java
- Windows - C#
If you are planning to develop a hybrid app means you need following skills
- HTML
- JavaScript
- CSS
Hybrid apps will work in all the platforms (IOS, Android, BB, Win etc). But the performance will be less when compared to native apps. It is called light weight apps. It is mainly used for approval kind of scenarios like Purchase order approval, PR approval etc.
Native apps will have the capability to work even the device is offline. You can perform operations even the device is offline (View data, update data etc). Once the device comes online the changes made will be updated to the SAP.
Native apps can be developed using SUP only, Netweavver Gateway only and NW Gateway+SUP (In this case SUP will be used only for Proxy configuration for more security). Offline capabilities will not be available in the last two case(In future it may be added).
Recently SAP released a group of apps called FIORI which is developed based on the web technologies like HTML, Javascript and SAPUI 5.
So there are lot of options to develop the app based on the requirement.
Hope it helps.