What Is the Primary Difference between Optimistic Rollups and ZK-Rollups?

Optimistic Rollups assume transactions are valid by default and use a fraud-proof system where a challenge period allows anyone to dispute an invalid transaction. ZK-Rollups, conversely, use zero-knowledge proofs (specifically SNARKs or STARKs) to cryptographically prove the validity of every off-chain transaction batch before it is submitted to the Layer 1 chain.

This fundamental difference results in ZK-Rollups having near-instant finality on Layer 1, while Optimistic Rollups have a delay due to the challenge window.