In procedural programming languages such as FORTRAN or Pascal, a program consists of a procedural part and a data part. The procedural part consists of the set of program instructions, and the data part, the numbers and character strings that are manipulated by the instructions. Programs typically contain several modules of instructions that perform computations on the same data set. When some change is made to the format of the data, every module that uses it must then be modified to accommodate the newly revised format. This places a heavy burden on the software maintenance process and makes these types of programs more prone to errors.

In an object-oriented system (OOS) the emphasis between data and procedures is reversed. Data becomes the primary object and procedures are secondary .For example, everything in the universe of an OOS is an object, and objects are inaccessible to outside procedures. This form of structuring is sometimes called Encapsulation or data hiding. It is a well known system design principle used to make systems more modular and robust. With encapsulation, objects are associated with their own procedures and, as such, are responsible for their own actions. Thus, when some change is required in the data or procedure, only the changed object need be modified.