Simply hold down the Command key and your mouse button and drag a menulet to a new position. The other Menu Extras will scoot out of the way as you drag. Release the mouse button to drop the menulet in its new position.
Removing is the same process as moving, with the exception of you drag downward and release the mouse. On release, a poof of smoke animation will appear, to let you know you have removed the item.
Apple provides Mac users with over two dozen handy menulets. Go to your startup disk and select System > Library > CoreServices > Menu Extras. The menulet names should give you an indication of their intended function. Double clicking a menulet will automatically add that menulet to the main menu.