What is the difference between an embolus and embolism?

An embolus is a physical mass (fat, air, blood clot, etc) which can travel to a different location in the arteries.

An embolism is when an embolus actually blocks a capillary.

A thrombus is a physical mass which remains attached to it's site of origin. A thrombus may detach to become a thrombo-embolus which may then cause an embolism.