The layman term for drain cover (grate) is manhole cover. Say "manhole cover" and images of spies/heros making use of the underground sewage system popping out of manhole cover in Hollywood films appear.
Historically since that time Man invented the sewage system since the Roman Times to drain water and urban waste away for sanitation purposes, manhole covers (drain covers or grates as we would prefer to call them) have been invented out of the basic need to prevent people from falling into underground sewage and to prevent large objects from entering the drains which cause choking which eventually could lead to flooding.
Fast forward to the present day, the sewage system is still utilised under all cities around Jonite channel drain covers . Instead of the crude stone manhole covers the Romans used in the ancient days, we now have special steel reinforced stone gratings that serve the thousand year old function of covering the inlets of our sewage system.
More than just a cover, check out Jonite's range of drain covers (drain gratings) here: http://www.jonite.us/products/creative-grates