Apache Cocoon is a Spring-based framework built around the concepts of separation of concerns and component-based development.
Cocoon implements these concepts around the notion of component pipelines, each component on the pipeline specializing on a particular operation. This makes it possible to use a Lego(tm)-like approach in building web solutions, hooking together components into pipelines, often without any required programming.
Cocoon used as web framework is "web glue for your web application development needs". It is a glue that keeps concerns separate and allows parallel evolution of all aspects of a web application, improving development pace and reducing the chance of conflicts. In particular it makes it easy to support multiple output formats, offers continuation based web controller implementations and comes with a JSR-168 compatible Portal implementation.