A Cattle Guard is like he said, trench dug out, with pipe layed across it, welded together on a frame. Usually about 8 feet across it, with spaces between the pipe of about 4 to 5 inches. Placed at entrance and exits in the place of a gate usually. Sometimes tho (in very few places) open grazing will have them on the roads. Cows have no depth perception, so they won't cross over the pipe.