The ssdeep project page describes it as a library for "...computing context
triggered piecewise hashes (CTPH). Also called fuzzy hashes, CTPH can match
inputs that have homologies. Such inputs have sequences of identical bytes in
the same order, although bytes in between these sequences may be different in
both content and length".

For an in depth paper explaining context triggered piecewise hashes please see
http://dfrws.org/2006/proceedings/12-Kornblum.pdf