it's the yellow button in the middle. It will drop the it down to the right side of the dock with it's own little icon. Simply click on the mini-icon to bring it back up.
Another way is apple(command)+H that will hide your active application (shown in by name in the upper left on the menu bar)
Option+Apple(command)+H will hide all in active applications.
Now with those two they will not drop to the right side of the dock in it's own mini icon. In order to relaunch the window you will have to click on the application icon on the dock.
There are several other solution to this. In lion you have multiple desktops you can swipe through with different programs active on each, you could also use hot corners. Hope these Help