Actions are central to WebWork (WW) as they are the controllers in your application. For example, lets look at a normal flow of events for a form post. A user enters information into a form and posts it. Using WW, you would post the form to an action (*.action) URI. The servlet container would see that all action URIs are mapped to the Dispatcher Servlet so the post would be sent there. The Dispatcher Servlet will retrieve the appropriate action based on the URI and build its context. After the context is built, the dispatcher will call the action's execute method. The action will perform its work and return a string to the dispatcher that it will use to determine what, if any, view should be rendered to the user. Most times you will return SUCCESS, ERROR, INPUT, or LOGIN from your action. You would then provide a view mapping entry which would map these to a view.
WW requires all actions to have a corresponding view. There are two exceptions to this rule. First exception is if your action is part of a chain, you do not need a corresponding view. Only the last action in the chain requires a view. Second exception is if your action returns NONE. This is a special token that WW uses to indicate to the dispatcher to not forward the request to a view. The assumption is that you have provided any request handling necessary such as respond.sendRedirect(). See redirect for an example.
The power in Action comes from its context. When an action is retrieved, it's context is built by the DefaultActionFactory. This class will chain together other factories that will each take part in setting the action's context if necessary. For instance, lets say your action was a Java action. The default action factory will delegate retrieving the action to the ParametersActionFactory which will delegate to the next factory which will delegate to the next one, etc. until the JavaActionFactory handles the call and returns the Java action. The ParametersActionFactory will then set all the action's setters based on the request parameters. The hierarchy of factories may or may not take part in setting this action's context. Some factories determine if they should take part in setting an action's context by examing a marker interface. WW provides several marker interfaces that an action may choose to implement. In addition, this hierarchy of factories allows you to create actions from Java, JSP, JavaScript, and XML files. But, most of the time your actions are Java files that extend ActionSupport.
For more information see Action's JavaDoc