Apache Cocoon is a web development framework built around the concepts of separation of concerns and component-based web 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 without any required programming.
Cocoon 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.