Decentralized Consensus:

Proof of Work (PoW): Initially, Ethereum used the PoW consensus mechanism (like Bitcoin). In PoW, miners compete to solve complex mathematical problems, and the first to solve it gets to add a new block to the blockchain and receive ETH as a reward.
Proof of Stake (PoS): Ethereum is transitioning to PoS, where validators replace miners. Validators are selected to add new blocks based on how much Ether they "stake" (lock up as collateral). This is more energy-efficient and allows for faster transaction processing.