Van der Waals force (London dispersion force):

Although the two forces were discovered by two different scientists (Johannes Diderik van der Waals and Fritz London) the two forces are similar and their names can be used interchangeably. Van der Waals force arises because in a electrons are mobile. Thus in a bond pair of electrons the two electrons may find themselves in one end of a molecule (although they are meant to be on opposite sides) and the other end is left empty. This causes a temporary negative charge on the electron rich side and a temporary positive charge on the electron deficient side. This momentary charge induces a opposite charge on the neighboring molecule and thus a temporary force of attraction. This sets of a chain reaction as one molecule induces charge on another molecule which in turn cause temporary charge in another molecule and this process continues.

Vander Waals force is very weak but is present in all substances and is the dominant force of attraction in gases.