The main thing to remember is that the include action allows both static and dynamic content to be included, whereas the include directive only allows static content. An include directive regards a resource like a JSP page as a static object; i.e. the text in the JSP page is included. An include action regards a resource like a JSP page as a dynamic object; i.e. the request is sent to that object and the result of processing it is included.
You could read "JSP.1.10.5 Including Data in JSP Pages" in the specification for more details.