When developping a web application, we all know that we have several parts to focus on ; presentation flow management, business rules development, data persistence, and so on... It is generally called N-Tiers architecture, N being function of the complexity of the application.
J5EE for instance aims to cover all the scope of web application concerns.
For the ones who don't want a all in one framework, or who develop with other languages than Java, it exists many "little" frameworks which focus on a particular part of the web application development problematics. It is the case especially of Struts, or Wickets, or Spring, etc...
This is in this context that the PureMVC framework occurs ; its scope is only to manage the presentation logics, ie how the developper has to design his application to manage views, actions, controls, ..., in the web pages. It is clearly a replacement of Struts and it is based uppon clear explained design patterns of the Gang of 4 : proxy, médiator, command, facade, observer ; five design patterns for building simple and well structured web applications from the presentation point of view.
The "plus" of the techno is that it has been derived for many languages : Php, Ruby, Java, Flex, ... We may even consider now developping an entire application in different languages (for instance Php and Java) with the same underluing concepts... It could be the case in a SOA approach, eg a web portal (public access) in Php which would focus on referencing, and in java/gwt for the business part of the application (private access).
Another interesting view is that it is ,by construction (it is only based uppon the use of design patterns, not libraries), compatible with other frameworks like Spring or Restlets...
So let imagine the following global architecture :)