Gesetz. Law, passed by the government; legally binding directive; an inherent way of doing something, an order of things; an unchangeable set of relations in nature.
Verordnung. A directive set by a governmental body.
Anweisung. A command, order; a written document communicating what is to be done.
Richtlinie. A guideline; an indication, recommendation, suggestion of how one should behave, handle a certain situation.
Regel. Rule. Latin regula, a standard, a set of measures; an agreement about behaviour, for how to do something; how in general something is done.
Leitplanke. Literally a guard rail along a road or highway; a guideline.