ABAP is the programming language of the SAP R/3 system. It is a proprietary language that is used to write both the fundamental SAP R/3 system itself and the standardized business applications that run on top of this system. It can be used for customer extensions of the system or entirely new software applications as well. The on-line language reference is available at no cost from the SAP Help Portal. Another great community - almost as rewarding as StackOverflow - is the SAP Community Network.