A French drain is primarily used for collecting subsurface water—water that is below ground.  It is a trench lined with filter fabric and has a pipe with holes in it placed on the bottom.  Gravel is used to backfill the trench.  Near the top of the trench, the fabric is folded over, totally enclosing the system.
It works when water moving across the trench falls through the spaces between the gravel, travels to the bottom of the trench, enters the holes in the pipe, and flows away from the area with the slope of the pipe.
It is important that the drainage contractor recognize when a French drain is needed as a drainage solution.  We commonly see surface drain systems composed of catch basins installed on the surface of the soil when a French drain was the best way to solve the drainage problem.