Classes are templates for objects. Conversely, you can say that the type of an object is the same as its class. A class is an abstract description of an object. You could say that it is a set of instructions for building an object. The attributes of objects are defined by the components of the class, which describe the state and behavior of objects.
Classes, their instances (objects), and access to objects using reference variables form the basics of ABAP Objects. These means already allow you to model typical business applications, such as customers, orders, order items, invoices, and so on, using objects, and to implement solutions using ABAP Objects.
However, it is often necessary for similar classes to provide similar functions that are coded differently in each class but which should provide a uniform point of contact for the user. For example, you might have two similar classes, savings account and check account, both of which have a method for calculating end of year charges. The interfaces and names of the methods are the same, but the actual implementation is different. The user of the classes and their instances must also be able to run the end of year method for all accounts, without having to worry about the actual type of each individual account.
ABAP Objects makes this possible by using interfaces. Interfaces are independent structures that you can implement in a class to extend the scope of that class. The class-specific scope of a class is defined by its components and visibility sections. For example, the public components of a class define its public scope, since all of its attributes and method parameters can be addressed by all users. The protected components of a class define its scope with regard to its subclasses. (However, inheritance is not supported in Release 4.5B).
Interfaces extend the scope of a class by adding their own components to its public section. This allows users to address different classes via a universal point of contact. Interfaces, along with inheritance, provide one of the pillars of polymorphism, since they allow a single method within an interface to behave differently in different classes.
Check these links for more details
http://help.sap.com/saphelp_47x200/helpdata/en/c3/225b6254f411d194a60000e8353423/frameset.htm
http://help.sap.com/saphelp_47x200/helpdata/en/c3/225b5c54f411d194a60000e8353423/content.htm
Bluesky
