Recently I deployed a web app in WebSphere Community Edition v3. Eclipse smartly created geronimo-web.xml and web.xml. In JSF 2.0 we learned about the use of web.xml but what is the use of geronimo-web.xml? Can I ignore this file and simply remove from the app?
It's a server-specific deployment configuration file which allows you to configure server-specific properties from inside the deployed web application, such as class loading, character encoding, JNDI resources, etcetera.
Other servers have also this kind of server-specific configuration files. Glassfish has glassfish-web.xml, Tomcat has context.xml, etc. IBM Websphere CE uses Apache Geronimo as container.
You can just remove it if you want, but why would you?
Please note that the web.xml is not specific to JSF API, but to the Servlet API. JSF has the faces-config.xml.