Defines an object that receives requests from the client and sends them to any resource (such as a servlet, HTML file, or JSP file) on the server. The servlet container creates the RequestDispatcher object, which is used as a wrapper around a server resource located at a particular path or given by a particular name.
I'm not sure they do. In each case they are talking about the behaviour, which is defined by the methods. While there will always be a concrete class required to do the actual work, we don't need to know what it ism, we only need to know how it behaves. That is, in this case it behaves like a RequestDispatcher.