Plain English
A rollup is a specific kind of Layer 2 that processes transactions off-chain and submits a compressed proof of those transactions back to the main chain.
How it actually works
Two main flavors. Optimistic rollups (Arbitrum, Optimism, Base) assume transactions are valid by default and allow a challenge period for disputes. ZK rollups (zkSync, StarkNet, Scroll) submit a cryptographic proof that the off-chain computation was correct — no challenge needed, faster final settlement, but more technically complex to operate.
What it means for you
When the curriculum talks about cheap Ethereum transactions, we mean rollups. The choice between optimistic and ZK affects how quickly you can withdraw funds back to mainnet.
We cover the practical decision: which rollup for which use case, withdrawal mechanics, and how to size liquidity across them.
Educational content only. Not investment, tax, or legal advice.