Basic structural evolution Miner (Bitcoin, old Ethereum): wins the right to add the next block by literally burning electricity on a race (proof-of-work). Decentralized by design — anyone with hardware can compete. Validator (modern Ethereum): same job — deciding what goes in the next block — but instead of winning by computation, you win by staking money as collateral. Cheaper on electricity, but the "right to add the next block" is now rotated among a known, registered set of participants rather than an open computational race. Sequencer (rollups like Linea, Arbitrum, Optimism): a further step removed. A rollup isn't its own independent blockchain — it's a layer that batches up lots of transactions cheaply, then periodically posts a compressed summary back down to real Ethereum, which is what actually secures everything. The "sequencer" is just whoever's currently responsible for deciding the order of transactions within that layer, before they get batched down. Almost every rollup today runs this as one single company-operated server — a deliberate, known, and openly acknowledged centralization tradeoff for speed, with plans (varying by project) to decentralize it later.