Assuming you mean Apache Struts and JSF - there is a huge difference between the Frameworks.
Struts is JSP and Tag centric and with a fixed dependecy on being deployed in a conventional Servlet HTTP request / response environment.
JSF is somewhat more generic. It is component centric and has no specific dependency on HTML markup / tags apart from the fact that the renders for most components are tailored for conventional browser output via HTML.
There is a lot of material out there comparing the frameworks, you've not stated which (if either) you are already familiar with so I'd suggest you start with the Struts Quickstart FAQ which you can find at http://struts.apache.org/
and then look at the JSF site at Sun and maybe Oracle as well:http://www.oracle.com/technology/tech/java/jsf.html