Rollup

A rollup is the leading design for scaling a blockchain without abandoning its security. Transactions execute on a separate, faster layer, then get bundled up and posted back to the main chain in compressed form, so the base chain still anchors what happened without processing every step itself.

Ethereum is where rollups matter most in practice. Networks such as Arbitrum, Optimism and Base run as rollups on top of it, and the user-visible difference is the gas fee: the same swap that costs a two-figure sum on Ethereum mainnet during a busy hour can cost pence on a rollup, because thousands of users share the cost of each batch posted back.

Two families exist, and one line covers the difference. Optimistic rollups assume batches are valid and allow a challenge window to dispute fraud, which is why withdrawals back to mainnet can take days. ZK rollups post a cryptographic validity proof with each batch, so there is nothing to wait out. Both inherit their security anchor from the chain underneath, which is the property that separates rollups from an ordinary sidechain running on its own consensus.

For a beginner the practical takeaways are that rollups have their own addresses and bridges, the coins on them are the same assets in wrapped or bridged form, and cheap fees change what is worth doing on-chain at all.