What are Van Der Waals dispersion forces?

Van der Waals forces are a form of intermolecular (between molecules) force. 

A covalent bond is created by a shared pair of electons. (For example two chlorine atoms will share 2 electrons to form a Cl₂ molecule). 

Within a covalent bonds, the electrons are not fixed, but constantly moving around the atoms. These may form temporary dipoles (even if the two atoms are of equal electronegativity such as Cl₂).

These temporary dipoles will attract their opposite dipole and repel the same dipole (ie. d⁺ attracts d^-). These random temporary dipioles are constantly forming, leading to attraction between molecules.

Van der Waals dispersion forces are considered a weak form of intermolecular force.