London Dispersion forces are caused by uneven distribution of electrons. Electrons are constantly moving around in an atom. When there are more electrons on one side of the nucleus than the other, a partial negative charge is produced where there more electrons and a partial positive charge is produced where the nucleus is as shown in the diagram below. 
In the next figure, the uneven charge distribution in one atom causes the electrons in a neighboring atom to be rearranged. This creates a weak attraction between the two atoms.
Source: Jennifer Roushar; Images from chem.purdue.edu
All molecules, whether they are polar or nonpolar, have London Dispersion forces. It is the only type of intermolecular force that exists between noble gases and nonpolar molecules.
Source: Jennifer Roushar
The larger the molecule, the greater the London dispersion forces. This is because larger molecules have a bigger electron cloud, thus creating a stronger temporary dipole when the electrons become asymmetrically distributed.
Bromine is a much larger molecule than fluorine. Because of this size difference, there are stronger London forces thus leading to a higher boiling point as shown in the table above.
Long, skinny molecules have stronger London forces than short, fat ones. This is because of the increase in the amount of surface area- more spots to temporarily attract to neighboring molecules. Propane, used in gas barbecue grills, contains only 3 carbons. Octane, found in gasoline, contains 8 carbons. Thus octane has stronger London forces and higher boiling points as shown in the table below.
