With the recent release of Ubuntu 10.04 Lucid, the new default theme is called Ambiance.
Ambiance introduced a movement of the window border controls from the top right side, to the top left side of the window.
Ambiance_R (Ambiance Right Side) is a modification of the default theme and moved the controls back to their original position.
This is a direct modification of the default theme. The original name of this theme is Ambiance, this version of the theme is called Ambiance_R (Ambiance Right Side).
No other changes have been made to this theme other than to restore the window border controls to their original right side position and reactivate the menu control in the original left side top position.
To install this them in Ubuntu 10.04 you can follow these steps.
System -> Preferences -> Appearance
Click the Theme tab.
Browse to the Ambiance_R.tar.gz file.
This will install the theme automatically for you.
Once installed you should now see a new theme entitled Ambiance_R.
Select this theme and you will have the new Ambiance theme with the controls in the top right side position.