http://www.galleryoffluidmechanics.com/vortex/am1260b.htm

The Ground Effect (or Wing in Ground Effect) is a natural phenomenon that occurs due to vortices caused by a difference in pressure between the two sides of a wing. This effect can be very dangerous to inexperienced pilots, but can be utilized by creative engineers.

This page is designed to provide the basic understanding of how the ground effect works.