ABAP is a programming language developed by SAP for the interactive development of application programming. ABAP/4 is a fourth generation language. ABAP/4 is tightly integrated across all the modules viz like SD, MM, HR etc and most or all of the modules and transactions(like screens are created using ABAP). All application programs, along with parts of the R/3 Basis system, are written in the ABAP Workbench using ABAP, SAP’s programming language. The individual components of application programs are stored in a special section of the database called the R/3 Repository. The R/3 repository serves as a central store for all of the development objects in the R/3 System.
Get an overview about Development Classes, transport organizer, Interfaces, Understanding your role as an ABAP developer, Dialog Programming, BDC, SAP Scripts, Enhancements and some of the ABAP syntax.
Download the manual on overview of ABAP.