How Does PoW Differ Fundamentally from Proof-of-Stake (PoS)?

PoW secures the network through energy expenditure and computational power (mining). PoS secures the network through economic stake, where validators are chosen based on the amount of currency they hold and are willing to "stake" as collateral.

PoW is about "work" to find a hash; PoS is about "holding" the asset and risking its loss (slashing) for malicious behavior.