Rule by decree allows the ruler to arbitrarily make laws, without any sort of legislative process or discussion. (See Reichstag Fire Decree as an example.) When no formal government institutions exist, rule by decree is common.
Some democratic leaders, such as the presidents of Russia and Mexico have the constitutional authority to issue decrees, as well.
The word decree (décret) is also used as a technical term to describe the executive decisions from the President or Prime Minister of France. Those decisions must conform to the Constitution and statutes of France, and it is possible to sue for their cancellation in the Conseil d'État.