The difference algorithm aims at comparing two sequences of objects and return an "edit script" which represents how one can transform the first sequence into the second sequence. The script describes the various insert object, delete object and keep object commands. The script is guaranteed to be the shortest possible in terms of number of commands.
From the script, one can either extract longest common sub-sequences (i.e. how similar the sequences are) or on the contrary the needed changes (i.e. how different the sequences are).