A DifferenceList is a way to express how a list differs from another list.
It is a concept quite common in languages with PatternMatching or Unification, such as PrologLanguage or OzLanguage.
For example, a list expressed as
Head = [a,b,c]
Tail = [c]
is equivalent to
difflist = [a,b]
since c is what you get subtracting tail from head.
Read http://homepages.inf.ed.ac.uk/pbrna/prologbook/node180.html for some explanation.