starter of this whole universe
SAP AG: www.sap.com
I would mention more than one link here. The place you find standard
help: help.sap.com. Some important
standards for programmers: Function
4 Abap Programmers
A nice undependent site with lots of content. It is written in german.
Some strange content here, maybe unapproved by SAP.
You can ask your questions in German and they will be quickly answered.