Ethereum

Ethereum is a public blockchain whose distinguishing feature is that it executes general-purpose programs. A smart contract is code deployed to the network, and any account can call it, which turns the chain from a payments ledger into a platform other applications are built on. The network is maintained by validators who stake ether and are rewarded for proposing and attesting to blocks, with penalties for misbehaviour. Because every node verifies every transaction, the state of the chain is agreed without a central operator. Fees are paid in ether and scale with the computational work a transaction requires. The tradeoffs are the reason alternative chains exist. Throughput is limited by the requirement that every node process every transaction, so fees rise when demand is high. Contracts are immutable once deployed unless written to be upgradeable, which means a bug can be permanent. And the account model puts the burden of key management on the user, where a lost key means lost funds.