A French drain uses gravity to direct excess water into a gravel-filled channel, where a buried pipe carries it away to a more suitable drainage area. The gravel allows water to fall easily to the drainpipe without leaving a large gap in the surface soil.
Since water always take the path of least resistance to the lowest level possible, a French drain is a great way to cure soggy spots in a lawn or prevent basement flooding. When water runs downhill on the surface and encounters the gravel-filled trench, the gaps between the rocks allow the stream to flow down to a perforated pipe at the bottom of the trench. As long as the trench and pipe run downhill at a shallow angle, the water flows along the pipe until it reaches the end.
A French drain is a longer-lasting solution than simply digging a drainage ditch, because eventually sediment refills the trench and requires re-excavation. The gravel fill in the French drain prevents that from happening, and also provides a level grade at the surface to minimize tripping hazards.
The French drain is not actually from France, but it was invented by Henry French of Concord, Massachusetts in the mid-19th century.