From the Cocoon website:
"Cocoon implements these concepts [separation of request from content & process] 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."
Basically, you map URL requests to pipelines.
Pipelines can do simple or complex "inline" processes.
Available at Apache's Cocoon Website