Trenches...
A trench is a type of excavation or depression in the ground that is generally deeper than it is wide (as opposed to a wider gully, or ditch), and narrow compared with its length (as opposed to a simple hole or pit).
In geology, trenches result from erosion by rivers or by geological movement of tectonic plates. In civil engineering, trenches are often created to install underground utilities such as gas, water, power and communication lines. In construction, trenches are dug for foundations of buildings, retaining walls and dams, and for cut-and-cover construction of tunnels. In archaeology, the "trench method" is used for searching and excavating ancient ruins or to dig into strata of sedimented material. In geotechnical engineering, trenches serve for locating faults and investigating deep soil properties. In trench warfare, soldiers occupy trenches to protect them against weapons fire and artillery.Link
So as it seems, the trench can mean different things to different people based on the field it is viewed from.
I remember in the movies, during a war scene, these trenches were mostly used to protect the soldiers.
They'll dig the trenches, and shoot from there to protect them from direct gun fire.
Trenches usually is below ground surface, and apart from the deliberately dug trench or the war part I mentioned. It is assumed to be caused by erosion.
As a slang where I come from, they call a place where poor folks come from trenches. It is a major slang over here. And truly the name suits some part of the rural areas.
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