Menachem Elon on the Difference Between Takannot and Gezerot

"It may therefore be said that, in the main, the term gezerah denotes legislation prohibiting the performance of a particular act that the Torah does not prohibit; the objective of a gezerah is to extend a prohibition established in the Torah, in order to decrease the likelihood that the Torah's prohibition will be violated.  Takkanah, on the other hand, generally denotes an enactment that imposes a duty to perform a particular act for the benefit and welfare of the community or any of its members.  These general rules, however, do not always apply,"  pg. 492 of Menachem Elon's "The Legal Sources:  Legislation, Custom, Precedent, Legal Reasoning.