A Cocoon developer works with Apache Cocoon, a web development framework built around the concepts of separation of concerns and component-based web development. They design and implement web applications using XML, XSLT, XPath, and SQL. Their responsibilities also include managing data transformation and presentation, setting up pipelines, and maintaining the server-side functionality of web applications. They ensure optimal performance, troubleshoot issues, and often collaborate with other developers to integrate different elements. They may also work on creating custom components and services within the Cocoon framework, following best practices to ensure the scalability and security of applications.