Software Engineering Overview
Let us first understand what software engineering stands for. The term is made of two words, software and engineering.
Software  is more than just a program code. A program is an executable code, which serves some computational purpose. Software is considered to be collection of executable programming code, associated libraries and documentations. Software, when made for a specific requirement is called  software product.
Engineering  on the other hand, is all about developing products, using well-defined, scientific principles and methods.

Software engineering  is an engineering branch associated with development of software product using well-defined scientific principles, methods and procedures. The outcome of software engineering is an efficient and reliable software product.
Definitions
IEEE defines software engineering as:
(1) The application of a systematic ,disciplined,quantifiable approach to the development,operation and maintenance of software; that is, the application of engineering to software.
(2) The study of approaches as in the above statement.
Fritz Bauer, a German computer scientist, defines software engineering as:
Software engineering is the establishment and use of sound engineering principles in order to obtain economically software that is reliable and work efficiently on real machines.
Software Evolution
The process of developing a software product using software engineering principles and methods is referred to as  software evolution.  This includes the initial development of software and its maintenance and updates, till desired software product is developed, which satisfies the expected requirements.

Evolution starts from the requirement gathering process. After which developers create a prototype of the intended software and show it to the users to get their feedback at the early stage of software product development. The users suggest changes, on which several consecutive updates and maintenance keep on changing too. This process changes to the original software, till the desired software is accomplished.
Even after the user has desired software in hand, the advancing technology and the changing requirements force the software product to change accordingly. Re-creating software from scratch and to go one-on-one with requirement is not feasible. The only feasible and economical solution is to update the existing software so that it matches the latest requirements.
Software Evolution Laws
Lehman has given laws for software evolution. He divided the software into three different categories:
S-type (static-type) -  This is a software, which works strictly according to defined  specifications and solutions.  The solution and the method to achieve it, both are immediately understood before coding. The s-type software is least subjected to changes hence this is the simplest of all. For example, calculator program for mathematical computation.
P-type (practical-type) -  This is a software with a collection of  procedures.  This is defined by exactly what procedures can do. In this software, the specifications can be described but the solution is not obvious instantly. For example, gaming software.
E-type (embedded-type) -  This software works closely as the requirement of real-world  environment.  This software has a high degree of evolution as there are various changes in laws, taxes etc. in the real world situations. For example, Online trading software.
E-Type software evolution
Lehman has given eight laws for E-Type software evolution -
Continuing change -  An E-type software system must continue to adapt to the real world changes, else it becomes progressively less useful.
Increasing complexity -  As an E-type software system evolves, its complexity tends to increase unless work is done to maintain or reduce it.
Conservation of familiarity -  The familiarity with the software or the knowledge about how it was developed, why was it developed in that particular manner etc. must be retained at any cost, to implement the changes in the system.
Continuing growth-  In order for an E-type system intended to resolve some business problem, its size of implementing the changes grows according to the lifestyle changes of the business.
Reducing quality -  An E-type software system declines in quality unless rigorously maintained and adapted to a changing operational environment.
Feedback systems-  The E-type software systems constitute multi-loop, multi-level feedback systems and must be treated as such to be successfully modified or improved.
Self-regulation -  E-type system evolution processes are self-regulating with the distribution of product and process measures close to normal.
Organizational stability -  The average effective global activity rate in an evolving E-type system is