The London dispersion force is a type of intermolecular force named for the physicist Fritz London. It occurs between molecules that do not have permanent dipoles (i.e. do not have a permanently positive and a permanently negative end), and is also known as induced dipole-dipole forces.
The electrons about a molecule of Bromine, Br-Br, are, in theory, evenly distributed around the two Bromine atoms. However, in practice, they are randomly scattered, meaning that there is a high probability that they are found more towards one end than the other. This creates a slight charge on both ends, which then causes other nearby molecules to form temporary dipoles as well. This charge creates a weak attraction between the molecules, which means, when the temperature is low enough, they will form liquids. The dipoles are temporary, unlike polar substances, which have a permanent dipole. The London dispersion force explain why non-polar solvents do not exist only as gas; due to the London dispersion force, they have a liquid and a solid form.