Proof-of-Work (PoW), the term was coined by Cynthia Dwork, formalized in a 1999 paper by Markus Jakobsson and Ari Juels.
There are two mainstream classes of proof-of-work protocol:
Proof of work is also known as mining.
Proof of work mean hardwork for computers.
Miners must compite to solve a difficult puzzle with their resources and help together by competing to build the public ledger.