The Myth of the Three-Layer Architecture
Conventional Wisdom holds that, when developing a web application, one splits the application into three distinct layers: the presentation layer, the business (or application layer), and the database layer. These don't quite line up with the three physical tiers of an application.
In a modern web application, the presentation layer is often structured after some variant of the MVC pattern. The model serves as the "back-end" of the presentation layer, the controller (or presenter) is the display logic and event loop, and the view is the HTML that gets sent to the browser. The presentation layer could in some sense be thought of as a three-layer architecture within a three-layer architecture. The presentation layer's idea of business logic, however, revolves around interpreting and acting on user-generated events (that is, after all, the business of a presentation layer). The "myth" of the three-layer architecture is that as applications become more and more user-centric, this presentation logic grows ever more important, rivaling even the business layer.
The problem with current web applications, then, is that the responsibilities of the presentation layer are actually spread across two distinct tiers: the web server and the web browser. From the user’s perspective, the UI logic happens "remotely." Thus the application is, from the user’s perspective, no better than CRUD screen access into a mainframe.