A French drain system is essentially a ditch filled with gravel to channel water runoff. A French drain may or may not have a pipe at the bottom of the trench. However it's constructed, its main purpose is to protect the foundations of buildings from being penetrated by draining water.
A basic French drain system consists of a narrow trench that's several feet deep. A small layer of gravel or small stones lines the bottom, followed by the placement of one or two perforated pipes known as weeping tile. Atop the pipe, the majority of the trench is filled with gravel. There may or may not be a layer of dirt or sod covering the trench. The pipes increase the amount of water that can be drained, while a wider trench further increases its ability to collect water runoff.
When rain falls, a functional French drain redirects the runoff, especially if a home or building sits at the bottom of a slope where it would otherwise be inundated by water. Sometimes gutter downspouts may feed directly into a French drain to help alleviate pressure on a storm drain.