In early WWW applications program code and HTML instructions were mixed inseparable in single program files. The demand of appealing designed WWW pages and the increasing complexity of modern WWW applications with more and more people involved do not allow such an approach any more. The Model-View-Controller Concept (MVC) is based on the separation of data processing, presentation and application logic.
This makes it possible that programmers and webdesigners can jointly work on the same project without interfering with each other - leaving logic and presentation separated. Small MVC based WWW projects can be implemented in Java with JSPs, Servlets and the standard libraries. For bigger projects modern Java frameworks like Struts and JavaServer Faces are needed. They are MVC based by design but their approach is different.