The V-model is a lifecycle model useful in the product development process that shows the link between the test specifications at each phase of development.  Simply put, it looks like this:

The name comes from the basic shape of the model.  The development phases start on the left side going down, from gathering and refining the requirements to implementation of the design.  The test effort goes along the right side from the bottom up, starting with unit testing and continuing until validation testing is correct.  The V-model twists the traditional waterfall model a bit by linking the test plans with the respective development phase.  For example, after the requirements are created, reviewed, and approved, a validation test plan can be developed.  This can also be a great tool to help refine the requirements so they are clear, understandable, and testable.  The Integration test plan is linked closely with the Architecture Design phase, where all the interfaces are defined.  The integration test plan focuses on ‘big block’ functions and collections of functionality.  The lowest test level, unit testing, is linked to the implementation phase.  Unit testing looks at the smallest functional levels of implementation to make sure they work correctly.

There’s not much more in terms of the model description, but the way you use the model will determine your success.