Model, View, Controller is a popular programming pattern which has forced you to design 3 decoupled core systems. If we take the PHP example of earlier, the model is what ever database we're fetching data from, the controller is the php framework and ORM e.g. Symfony with Doctrine, which perform all the business logic with the fetched data. Html, css and javascript perform the view portion. The view portion can also be done via a templating engine, but should still be designed to not depend on the controller logic. The importance of MVC is to design systems that, if needed could be completely redone without needing to perform major rewrites or even better refactoring on the other 2 systems.