Subject to where you are from, different countries have different terms for "storm drain".
Accordingly to Wikipedia, a storm drain is basically a tunnel that carry runoffs (rain) from urban areas. For a layman, it helps to think along the line of sewage system operates beneath our cities.
In america, this is typically referred to as "storm drain" or "storm sewer".
In great britan, this is often called "surface water drain" or "surface water sewer".
In Australia and New Zealand, it is best known "stormwater drain".
And there you have it, next time you hear someone mentioning "storm drain", "storm sewer", "surface water drain","surface water sewer" and "stormwater drain", floor grilles and registers know they mean the exact same thing.
Functions of storm drain include draining excess water (rain or storm water) and ground water from pavement, streets and carparks into rivers and streams to counteract flooding.
Although storm drains perform a amazing job of draining excess water from the urban areas, the inlets are essentially openings where left uncovered could be a hazard to pedestrians. Left uncovered, rubbish will get into the storm drains and cause choking which may eventually lead to flooding.
To guarantee storm drains serve their purposes while minimizing the potential risks posed to pedestrians and to prevent choking of the drains, drain covers (gratings) are widely-used to cover the inlets (openings) of the storm drains to accomplish the two purposes.
Over at Jonite, we concentrate on steel reinforced drain covers (gratings) that cover your storm drains while preserving the aesthetic elements of the architecture around the drain covers.