What is Rollup?

A rollup is the dominant design for an Ethereum layer 2. It executes transactions on its own network, bundles many of them together, and posts a compressed record of the batch to Ethereum, where the record becomes final and inherits Ethereum’s security. Users get fees that are a small fraction of mainnet, and the base chain gets the finality without doing the execution.

Two families exist. Optimistic rollups, Arbitrum and Optimism among them, post batches and assume they are valid unless someone proves otherwise within a challenge window, which is why withdrawals from them to Ethereum can take days. Zero-knowledge rollups, zkSync Era, Linea and Scroll among them, post a cryptographic proof that the batch is valid, so nothing needs a challenge window and withdrawals can be faster once the proof is verified. The compressed data posted to Ethereum is what makes a rollup a rollup: anyone can reconstruct the full state of the layer 2 from the base chain alone, which is the security property sidechains lack.

For an account holder the rollup is mostly invisible. Addresses are the same 0x strings as on Ethereum, tokens follow the same standards, and the same wallet works everywhere. What changes is where a balance lives: a token bridged to a rollup sits at your address on that rollup, and a portfolio that ignores the rollups is missing every balance that moved there for cheaper fees.

TrueHold reads rollup balances and positions from the same address as Ethereum, and shows each rollup as its own slice of one total. Bridges between Ethereum and a rollup are recorded as transfers, so moving between layers never looks like a sale.

Withdrawing back to Ethereum is where the two families feel different. An optimistic rollup makes you wait out the challenge window, typically about a week, unless a third-party bridge fronts the funds for a fee; a zero-knowledge rollup releases funds once its proof has been verified on Ethereum, which is hours rather than days.

For most users the difference only matters on the day they move funds back to mainnet.