A French drain is a trench filled with clean stone, usually containing a perforated pipe and wrapped in geotextile, used to intercept and carry away groundwater or surface run-off. It is commonly used around buildings, along sloping ground and beside driveways to keep water away from structures.
Water flows readily through the voids in clean stone, so a stone-filled trench acts as a preferential path that collects water and conveys it to an outfall. A perforated pipe in the base increases capacity, and a geotextile wrap keeps soil fines out of the stone so it does not silt up.
A French drain intercepts and moves water; a soakaway stores and infiltrates it. They are often used together, with a French drain collecting run-off from sloping ground and discharging into a soakaway, attenuation system or watercourse.