Verification is done by frequent evaluation and meetings to appraise the documents, policy, code, requirements, and specifications. This is done with the checklists, walkthroughs, and inspection meetings. Validation is done during actual testing and it takes place after all the verifications are being done.

Verification is the checking of or testing of items, including software, for conformance and consistency with an associated specification. Software testing is just one kind of verification, which also uses techniques such as reviews, inspections, and walkthroughs. Validation is the process of checking what has been specified is what the user actually wanted.

• Verification: Have we built the software right? (i.e. to specification)
• Validation: Have we built the right software? (i.e. Is this what the customer wants?)

Ref:- http://en.wikipedia.org/wiki/Software_testing

The verification theory (of meaning) is a philosophical theory proposed by the logical positivists of the Vienna Circle. A simplified form of the theory states that a proposition's meaning is determined by the method through which it is empirically verified. In other words, if something cannot be empiricially verified, it is meaningless. For example, the statement "It is raining" is meaningless unless there is a way whereby one could, in principle, verify whether or not it is in fact raining. The theory has radical consequences for traditional philosophy as it, if correct, would render much of past philosophical work meaningless, for example metaphysics and ethics.

The word validation has several uses:

• In general, validation is the process of checking if something satisfies a certain criterion. Examples would include checking if a statement is true (validity), if an appliance works as intended, if a computer system is secure, or if computer data are compliant with an open standard. Validation implies one is able to testify that a solution or process is correct or compliant with set standards or rules.
• Validation should not be confused with verification. Validation can sometimes be measured by applying procedural objectivity, but verification is usually subjective$mdash;a judgement call. An easy way of recalling the difference between validation and verification is that validation is ensuring "you built the right product" and verification is ensuring "you built the product right."
• Validation can mean to declare or make legally valid or to prove valid or confirm the validity of data, information, or processes:
  • Validation of foreign studies and degrees.
• In computer programming terminology, validation refers to the process of controlling that data inserted into an application satisfies pre determined formats or complies with stated length and character requirements and other defined input criteria. Verification usually requires human judgement.
• In computer security, validation also refers to the process of verifying that a user or computer program is allowed to do something.
• In psychology and human communication, validation is the communication of respect for a communication partner which involves the acknowledgement that the other's opinions are legitimate.
• In the pharmaceutical and biotechnology industry, validation refers to establishing documented evidence that a process or system, when operated within established parameters, can perform effectively and reproducibly to produce a medicinal product meeting its pre-determined specifications and quality attributes (from European Union Good Manufacturing Practices Guide, Annex 15). Regulatory bodies in the U.S., European Union, and Japan (amongst many others) require validation, causing it to become its own sub-industry supporting the pharmaceutical, biotechnology, and medical device industries.
• in finance, validation is a process part of the "trade life-cycle."
• Validation is important because it disallows data that can not possibly be either true or real to be entered into a database or computer system.
• Validation against an incomplete or insufficient set of criteria can lead to a state of "validated" where "validated" does not confer the confidence that the term intends. Thus validation of the validation criteria is an important aspect that is often overlooked. Establishing such validation criteria can be a very difficult task when evaluating complex systems such as Air Traffic Management systems. Establishing "fitness for purpose" is often a more useful concept to support evaluation of complex systems in that the approach focuses on involving stakeholders in establishing and reviewing the purpose that the system must satisfy as the system emerges from early design. This allows flexibility in the evaluation process as ideas turn into detailed designs. Such flexibility is essential in the early development phases in order to avoid engineering white elephants. [The European Operational Concept Validation Methodology E-OCVM provides an approach to validating complex Air Traffic Management systems by establishing fitness for purpose in a world of shifting and incomplete validation criteria]E-OCVM.