The marketing says...
Apache Cocoon is an XML publishing framework that raises the usage of XML and XSLT technologies for server applications to a new level. Designed for performance and scalability around pipelined SAX (SimpleApiForXml) processing, Cocoon offers a flexible environment based on the SeparationOfConcerns between content, logic and style. A centralized configuration system and sophisticated caching top this all off and help you to create, deploy and maintain rock-solid XML server applications.http://cocoon.apache.org/
Cocoon lets you do pipelining of web services components and tries to separate out as much as possible content from presentation. Delivery of web content is defined by a "sitemap", which expresses (in XML of course) how URLs map to content, in terms of:
Generators, which create XML as SAX events;
Transformers, which transform SAX events into other SAX events (eg. XSLT);
Serializers, which convert SAX events into binary or character streams (eg. HTML) for consumption;
Selectors, which allow conditional processing (if, then, else);
Matchers, which match patterns to resources; and
Actions, which manipulate runtime parameters based on request and application state.