Wikipedia
The V-model is a term applied to a range of models, from a conceptual model designed to produce a simplified understanding of the complexity associated with systems development to detailed, rigorous development lifecycle models and project management models.
There are several radically different forms of the V-model, and this creates considerable confusion. The V-model falls into three broad categories.
Firstly there is the German V-Model "Das V-Modell", the official project management methodology of the German government. It is roughly equivalent to PRINCE2, but more directly relevant to software development. The key attribute of using a "V" representation was to require proof that the products from the left-side of the V were acceptable by the appropriate test and integration organization implementing the right-side of the V.
In the UK and throughout the testing community worldwide, the V-model is widely seen as a vaguer illustrative depiction of the software development process as described in the ISTQB Foundation Syllabus for software testers. There is no single accepted definition of this model, which is more directly covered in the alternative article on the V-Model (software development). There are therefore multiple variations of this version. This problem must be borne in mind when discussing the V-model.
The US also has a government standard V-model which dates back about 20 years like its German counterpart. Its scope is a narrower systems development lifecycle model, but far more detailed and more rigorous than most UK practitioners and testers would understand by the V-model.
In software development, the V-model represents a development process that may be considered an extension of the waterfall model, and is an example of the more general V-model. Instead of moving down in a linear way, the process steps are bent upwards after the coding phase, to form the typical V shape. The V-Model demonstrates the relationships between each phase of the development life cycle and its associated phase of testing. The horizontal and vertical axes represents time or project completeness (left-to-right) and level of abstraction (coarsest-grain abstraction uppermost), respectively.