Canon law is the body of laws and regulations made by ecclesiastical authority (Church leadership), for the government of a Christian organization or church and its members. It is the internal ecclesiastical law governing the Catholic Church (both Latin Church and Eastern Catholic Churches), the Eastern and Oriental Orthodox churches, and the individual national churches within the Anglican Communion. The way that such church law is legislated, interpreted and at times adjudicated varies widely among these three bodies of churches. In all three traditions, a canon was originally a rule adopted by a church council; these canons formed the foundation of canon law. 

Greek kanon / Ancient Greek: κανών, Arabic Qanun / قانون, Hebrew kaneh / קנה, "straight"; a rule, code, standard, or measure; the root meaning in all these languages is "reed" (cf. the Romance-language ancestors of the English word "cane"). 

The Apostolic Canons or Ecclesiastical Canons of the Same Holy Apostles is a collection of ancient ecclesiastical decrees (eighty-five in the Eastern, fifty in the Western Church) concerning the government and discipline of the Early Christian Church, incorporated with the Apostolic Constitutions which are part of the Ante-Nicene Fathers In the fourth century the First Council of Nicaea (325) calls canons the disciplinary measures of the Church: the term canon, κανὠν, means in Greek, a rule. There is a very early distinction between the rules enacted by the Church and the legislative measures taken by the State called leges, Latin for laws.
what is it?
governing the Catholic Church (both Latin Church and Eastern Catholic Churches), the Eastern and Oriental Orthodox churches, and the individual national churches within the Anglican Communion