Glossary
Layer 2
A protocol that runs on top of a Layer-1 blockchain to increase throughput or lower fees while inheriting the L1's security via proofs or fraud bonds.
A Layer 2 is a separate execution environment that posts its data or proofs back to a Layer 1 such as Ethereum. Users transact on the L2 — cheap, fast — and the L1 settles the result periodically.
Two dominant flavors: optimistic rollups (assume L2 blocks are valid, challenge them with fraud proofs during a withdrawal window) and ZK rollups (prove validity cryptographically up front). Both inherit L1 security: as long as the L1 is honest and the L2 contracts are correct, L2 assets can always be withdrawn.