AjaxUpdateContainer is a particularly cool component that allows any component on your page to be refreshed in the background without a full page refresh. AjaxUpdateContainer uses a WOComponentContent. All content between its webobjects tags will be refreshed when it is ordered.
If your AjaxUpdateContainer contains component actions, you must include the ERExtensions framework in your application and extend ERXApplication and ERXSession as described in the Project Wonder Quickstart guide. If all of your actions are Direct Actions inside the update container, this is not necessary.
Additionally, if you intend to use the "observeFieldID" binding, you must also include the ERExtensions framework (as described above). The requirement is because a partial form submit is necessary (to send the modified observed field back to the server), and WOForm required a change to be able to recognize that it should takeValuesFromRequest for the observed value.
A series of demos of AjaxUpdateContainer is available in the AjaxExamples project in Project Wonder.