ZK is an open-source Ajax Web application framework, written in Java, that enables creation of graphical user interfaces for Web applications with little required programming knowledge.The core of ZK consists of an Ajax-based event-driven mechanism, over 123 XUL and 83 XHTML-based components, and a mark-up language for designing user interfaces. Programmers design their application pages in feature-rich XUL/XHTML components, and manipulate them upon events triggered by end user's activity.