Rollups (Optimistic & ZK)
A rollup executes transactions off-chain and posts compressed data plus a proof to the main chain, which secures it.
In depth
Optimistic rollups assume batches are valid and allow a challenge window (usually about a week) during which anyone can prove fraud. Zero-knowledge (ZK) rollups post a cryptographic validity proof with every batch, so withdrawals are faster and no challenge period is needed. Both dramatically reduce per-transaction cost.
Why it matters
Rollups are the dominant Ethereum scaling strategy; their design explains withdrawal delays and fee differences.