ABAP Programming (BC-ABA)
This documentation describes how to write application programs within the three-tier client/server architecture of the ABAP application server.
ABAP applications are written in the ABAP programming language, and run within the application layer of the SAP Web Application Server.
ABAP programs communicate with the database management system of the central relational database (RDBMS) and with the presentation layer. A SAP-specific graphical user interface (SAP GUI) for users logged on to a SAP System and Web browsers for Web-based applications are available to you as presentation layers.
The documentation is divided into five sections:
Introduction to ABAP
This contains the basics of ABAP application programming. This information is essential for an understanding of ABAP programming. Following an overview of the ABAP side of the SAP Web Application Server, it introduces the essential features of ABAP programs and the ABAP programming language. Finally, it gives a short introduction to how you can create ABAP programs in the ABAP Workbench.
The ABAP Programming Language
This section describes the statements in the ABAP programming language. Beginning with simple statements for data declarations, data processing, and program flow control, it progresses to topics such as modularization and special techniques, explaining which ABAP statements can be used for which purposes.
ABAP User Dialogs
This section describes the SAP GUI-specific screens whose creation and use is integrated in the ABAP programming language.
Running ABAP Programs
This section explains how ABAP programs are executed on the application server. It shows how you can start ABAP programs, the conditions under which you must start them, and the different kinds of program execution.
ABAP Database Access
This section explains how to work with the central database in the SAP System. It describes the parts of the programming language that are converted into SQL statements in the database, and shows how you can program database updates.
This is an introduction to ABAP Objects, the object-oriented extension of ABAP. Objects, classes, and interfaces are introduced as the fundamental elements of ABAP Objects. It is shown how you can define classes yourself or by using interfaces or inheritance. This section also covers the handling of methods and events as components of classes.
Source : http://help.sap.com