What is the difference between a notification and a circular?

A notification is issued by a government (central/ state) to exercise the power of a legislative enactment (Parlimentary/ Assembly). These powers are in abundance in taxation (direct/ indirect) legislations. These notifications generally lay down the law taking care of some procedural aspects of the enactment. The circulars are issued by the chief executive in charge of the taxation hierarchy, and are normally explanatory/ interpretative or relax the rigours of the law administered.