Panel Class:

The panel class is a concrete subclass of the Container class. Panel class provides an intermediate level for Graphical User Interface organization. The panel is recursively nested, concrete container which is not a top-level window. That also does not have a title, borders or menus. It is thus ideal for packing other components and panels to build component hierarchy archies by using the inherited add ( ) method

Applet Class:

An Applet class belongs to java.applet package.  This is a subclass of the Panel class and therefore inherits its functionality.  An applet is a specialized panel which is used to develop programs than run in a web browser.