Proof-of-Work: ( $BTC , formerly $ETH ): Security comes from external physical cost. Miners compete to solve energy-intensive puzzles. To attack the network, you'd need to outspend the entire mining industry in hardware and electricity, making it economically irrational. Security is literally burned as energy.

Proof-of-Stake: (Ethereum, $SOL ): Security comes from internal financial stake. Validators lock up (stake) their own crypto as collateral. To attack the network, you'd need to acquire a majority of the staked asset, which would be astronomically expensive and likely crash the value of your own holdings. Attackers risk their own capital through "slashing" penalties.