A helper application is different in that it is started and runs as a separate application, in its own window and using its own memory space. Helper applications are complete programs that Navigator runs when necessary. One advantage of helper application over a plug-in is that you can multitask between a running helper application and the Navigator main window. Files read by helper applications can be downloaded and viewed within their own application window after your close Netscape Navigator. Remember that Netscape Navigator has no control over the behavior of a helper application once it starts.
Navigator only has the ability to start the application and display the appropriate file.