xApp Definition

Hi,

after a long time of engaging with caf I'm certain that it is possible to create xApps with CAF (of course therefor the certification of SAP is nessasary). But my questions now is, if it is possible to create xApps without CAF, too? For example xRPM was release before CAF was available. Moreover I've received a mail of an SAP employee who have written that its possible to realised xApps in ABAP, too. But how? What is the exact defintion of SAP xApps? What are the development tools and qualities of SAP xApps?

Thanks a lot!

Martin