Plain English
A rollup is an L2 that bundles transactions, executes them off-chain, and posts a summary to L1. Optimistic rollups assume everything is fine and let anyone challenge bad behavior within 7 days. ZK rollups post a cryptographic proof of correctness immediately.
How it actually works
Optimistic rollups (Arbitrum, Optimism, Base) have faster developer ergonomics — they run an EVM nearly identical to Ethereum mainnet, so existing contracts deploy with no modification. The 7-day challenge period is the trade-off: withdrawals to L1 take 7 days unless you use a third-party fast bridge. ZK rollups have shorter withdrawal times (often under an hour) but more complex tooling and longer-term advantages in privacy and finality.
What it means for you
For most users, the practical difference is withdrawal speed and the bridge ecosystem around each L2. The biggest L2s — Arbitrum, Optimism, Base — are all optimistic and have mature liquidity. ZK L2s are growing fast and will likely dominate by late-decade as proving costs fall. Both are safer than sidechains by a wide margin.
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Educational content only. Not investment, tax, or legal advice.