<f:ajax event="..." render="..." execute="..." listener="..." ...
event is a string indicating on which events an Ajax request will be fired,
render is a string indicating the identifiers of the components to render
execute is a string indicating the identifiers of the components to be processed on the server
listener indicates a method to invoke during the Ajax request
Complete documentation on the f:ajax tag can be found here: http://javaserverfaces.java.net/nonav/docs/2.0/pdldocs/facelets/f/ajax.html.
Here is an example of how to use the f:ajax tag to achieve PPR of an af:outputText based on a value change event originating from an af:inputText component:
<f:ajax name="change" render="ot" execute="@this" />
<af:outputText id="ot" ...
The @this value for the execute attribute indicates that only the component that triggered the Ajax request will be processed on the server.