How can I achieve page-up and page-down in OS X? I am using MacOSX Snow Leopard.

How do I achieve page-up and page-down?
If you have a full keyboard you can use the pgup and pgdown keys on your keyboard, near the numpad. 

If you are not using a full keyboard, function, labeled fn on your keyboard, plus the up and down arrow keys will give you a page up and down.

For certain applications, particularly in shell / terminal / tty windows, the expected behaviour is achieved with fn+shift+arrow up/down