PoW is a protocol that requires proving that work of some kind was successfully done. This algorithm rewards miners who solve mathematical problems to validate transactions and create new blocks. Miners must do the work before their blocks are accepted by others. The primary goal of PoW is securing cryptocurrency network and preventing cyber-attacks.
The system uses Bitcoin. The idea of this algorithm was originally published in 1993, yet the term PoW appeared later in 1999. The proof-of-work can be considered as the most prominent idea behind Bitcoin as it allows trustless and distributed consensus. However, the main drawback of PoW is high energy consumption. Another concern is that in case a currency's price or reward amount drop down, then fewer people have the motivation to mine. Hence, the security of the system will be significantly reduced.