These are the forces that holds individual molecules together in solids and liquids. They are very weak (relatively) forces which act over a very short range. They were discovered by Johannes Diderik van der Waals (a dutchman) in the mid to late 19th century.

It has recently been proposed that it is actually the Van der Waals forces between microscopic hairs/suckers on their feet and the surface they are on that allows Geckos to climb on any surface.