Hi Selvas,
I suggest you have a look at the API of the java.awt.Component class since everything is well described there.
But in short, paint is called everytime a component (or a part of it) needs to be painted. repaint will tell the GUI
thread to schedule a repainting of the component as soon as it can (invoking multiple times repaint will not necessarely repaint the component the number of times the method was invoked) . update will first erase everything painted on the component and repaint it cleanly.
Hope this helps a lil',
Val