Java applets inherit significant functionality from the Applet or JApplet class, including the capabilities to communicate with the browser and present a graphical user interface (GUI) to the user.
http://docs.oracle.com/javase/tutorial/deployment/applet/subclass.html
The Applet class provides a framework for applet execution, defining methods that the system calls when milestones occur.
http://docs.oracle.com/javase/tutorial/deployment/applet/appletMethods.html