Verification vs Validation

Following is an extract from the book PDCA/TEST: A Framework for software Testing, William E. Lewis. I found it quite good.

Verification vs. Validation 

Verification is proving that a product meets the requirements specified during previous activities carried out correctly throughout the development life cycle, while Validation checks that the system meets the customer's requirements at the end of the life cycle. It is a proof that the product meets the expectations of the users, and it ensures that the executable system performs as specified. The creation of the test product is much more closely related to validation than to verification. Traditionally, software testing has been considered a validation process, i.e., a life cycle phase. After programming is completed, the system is validated or tested to determine its functional and operational performance.

When verification is incorporated into testing, testing occurs throughout the development life cycle. For best results, it is good practice to combine verification with validation in the testing process. Verification includes systematic procedures of review, analysis, and testing, employed throughout the software development life cycle, beginning with the software requirements phase and continuing through the coding phase. Verification ensures the quality of software production and maintenance. In addition, verification imposes such an organized, systematic development practice that the resulting program can be easily understood and evaluated by an independent party.

Verification emerged about 15 years ago as a result of the aerospace industry's need for extremely reliable software in systems in which an error in a program could cause mission failure and result in enormous time and financial setbacks, or even life-threatening situations. The concept of verification includes two fundamental criteria. First, the software must adequately and correctly perform all intended functions. Second, the software must not perform any function that either by itself or in combination with other functions can degrade the performance of the entire system. The overall goal of verification is to ensure that each software product developed throughout the software life cycle meets the customer's needs and objectives as specified in the software requirements document.

Verification also establishes tractability between the various sections of the software documentation and the associated parts of the requirements specification. A comprehensive verification effort ensures that all software performance and quality requirements in the specification are adequately tested and that the test results can be repeated after changes are installed.
Verification is a "continuous improvement process" and has no definite termination. It should be used throughout the system life cycle to maintain configuration and operational integrity.

Verification ensures that the software functions as intended and has the required attributes, e.g., portability, and increases the chances that the software will contain few errors (i.e., an acceptable number in the final product). It provides a method for closely monitoring the software development project and provides management with a detailed status of the project at any point in time. When verification procedures are used, management can be assured that the developers follow a formal, sequential, traceable software development process, with a minimum set of activities to enhance the quality of the system.

One criticism of verification is that it increases software development costs considerably. When the cost of software throughout the total life cycle from inception to the final abandonment of the system is considered, however, verification actually reduces the overall cost of the software.
With an effective verification program, there is typically a four-to-one reduction in defects in the installed system. Because error corrections can cost 20 to 100 times more during operations and maintenance than during design, overall savings far outweigh the initial extra expense.