A french drain is a simple but extremely effective way to direct water on a course you choose. Most commonly used by homeowners in grass and garden settings, a french drain consists of a small trench, ideally dug with a contractor-grade trenching machine and lined with landscape fabric, and a perforated, corrugated pipe that runs the length of the trench. During installation, once the pipe is properly situated, the trench is typically filled with gravel, rocks, and/or dirt, and then covered in a manner of the homeowner's choosing - including grass/turf.
Many drainage problems are caused by ground water having no place to go once it has reached a roadblock or a low point. The french drain gives the water an easy path of egress, as the water permeates the rock and pipe and drains in whatever direction the drain is pointing. French drains do not help with surface water directly, though they do collect and direct water once it penetrates the surface and becomes groundwater.
Some people use the need for a french drain as an opportunity to install custom high-end rock and creek beds for an upscale designer touch. These hard landscape features are both beautiful and functional.
While no french drain (or any solution short of a dome for that matter) can guarantee a fully dry yard immediately after a major downpour, a properly installed french drain will ensure that water does not accumulate in low spots, corners, or against your home's foundation, and that your outdoor spaces are livable and usable to the greatest possible degree.