A blockchain is only useful if every participant in the network agrees on the same version of the transaction history. A consensus mechanism is the set of rules that lets a decentralized network — with no central authority — reach that agreement reliably.
Proof of Work (PoW), used by Bitcoin, has miners compete to solve a computationally expensive puzzle. The first to solve it gets to add the next block and earn the block reward. This competition requires real-world energy expenditure, which is precisely what makes rewriting history prohibitively expensive for an attacker.
Proof of Stake (PoS), used by Ethereum since its 2022 transition, instead selects block validators based on the amount of cryptocurrency they have locked up ("staked") as collateral. Validators who act dishonestly can have a portion of their stake destroyed ("slashed"), which replaces energy expenditure with economic risk as the security mechanism.
PoS networks generally consume far less energy than PoW networks, since there's no computational race to win. The tradeoff is a different, more complex set of economic and game-theoretic assumptions about validator behavior.
Neither mechanism is universally "better" — they represent different tradeoffs between energy use, decentralization, and the specific attack models each network is designed to resist.
