Why are they called ‘dispersion forces’ if they are an attractice force?
Although Danny Rodriguez has already excellently exposed what the dispersion force is in simple terms, the word dispersion still demands a better explanation in my opinion.

According to Wikipedia:


  The London theory has much similarity to the quantum mechanical theory of light dispersion, which is why London coined the phrase "dispersion effect."
  In physics, the term "dispersion" describes the variation of a quantity with frequency, which is the fluctuation of the electrons in the case of the London dispersion.


So the word "dispersion" here is a mere analogy.
The original work is this one (in German).