A French drain (also known as land drain, perimeter drain, French ditch, sub-surface drain, sub-soil drain or agricultural drain) is a trench filled with gravel or rock or containing a perforated pipe.
Its purpose is to redirect surface water and groundwater away from an area, so prevent it from penetrating or damaging building foundations. It may also be used to distribute water from, for instance, a septic drain field, or behind a retaining wall to relieve ground water pressure.
Modern French drain systems use perforated pipe surrounded by sand or gravel and geo- or landscaping textile to stop dirt and roots from entering and clogging the drainage pipe. The pipe provides a minor underground storage volume but the prime purpose is for the perforations to drain the area along the full length of the pipe and to discharge any surplus water at the end.