I need to hide the hud of StarCraft II during play so I can take screenshots of the full play ground. How I can do that?
You can only hide the UI during active game play if you are the observer or watching a replay file, not if you are the active player. The key command to do this is Ctrl+W.
If you are just trying to take screenshots of the map itself you can also do this from outside the game by opening up the map in the Starcraft 2 map editor.