At the beginning of your upkeep, for each land target player controls in excess of the number you control, choose a land that player controls, then the chosen permanents phase out. Repeat this process for artifacts and creatures. (While they're phased out, they're treated as though they don't exist. They phase in before that player untaps during their next untap step.)
(04/10/2004)
In multiplayer games, you can choose a different target player each turn. You can even choose yourself.
(01/10/2009)
You perform the selection and phasing out on lands first, then on artifacts, then on creatures. This means that if you choose an artifact creature during the second pass, your opponent will control one fewer creature when it comes time to perform the third pass.
