Optimistic rollups assume transactions are valid and rely on fraud proofs: any watcher can challenge a posted state root during a challenge window (typically 7 days on Arbitrum and Optimism). ZK rollups instead submit a validity proof (SNARK or STARK) with every batch, so the L1 contract verifies correctness before accepting the state root — no challenge window needed.

The economic and UX consequences diverge sharply:

Optimistic: cheap proving, but withdrawals to L1 wait out the challenge window unless a third-party bridge (e.g., Hop, Across) fronts liquidity.
ZK: expensive proving (GPU/ASIC farms), but near-instant L1 finality and smaller trust assumptions.
Both inherit L1 data availability in the canonical design; the difference is when correctness is proven.

Property	Optimistic	ZK
Proof type	Fraud (interactive)	Validity (succinct)
L1 withdrawal delay	~7 days	Minutes
Prover cost	Low	High