A rollup is a Layer 2 blockchain that processes transactions off the main chain and periodically posts a batch with a proof of correctness back to L1. The L1 verifies the proof (or accepts it under a fraud-proof window) and stores the compressed result. Users get L2 speed and cost; L1 stores enough data to reconstruct L2 state if the L2 sequencer disappears.
Rollups split into two main types based on how they prove correctness: optimistic rollups assume transactions are valid by default and let anyone submit a fraud proof during a 7-day challenge window; ZK rollups include a cryptographic validity proof with each batch, eliminating the challenge window. ZK rollups are mathematically more elegant; optimistic rollups are easier to build.
All major Ethereum rollups follow this pattern. Differences are in proof system, sequencer design, data availability layer, and EVM compatibility.
Rollups are the dominant scaling path for Ethereum. By 2026, the majority of Ethereum-network transactions happen on rollups, not on L1 itself. Understanding the difference between optimistic and ZK is necessary for working in DeFi, NFTs, or anything on-chain.
Base (the L2 TerminalFeed uses for USDC payments) is an Optimism-style optimistic rollup. The exchange-flows premium endpoint tracks flows on both L1 and L2.