The Core module is not tied to any web framework - it depends only on PicoContainer and optionally on PicoContainer Script.
The JSF module provides support for JSF.
The Struts module provides support for Struts 1.x.
The Struts2 module provides support for Struts 2.x.
The WebWork module provides support for WebWork 1.x.
The WebWork2 module provides support for WebWork 2.x.