A rollup is a Layer 2 scaling approach documented by Ethereum. It executes transactions outside Ethereum Layer 1 and posts data or results to Layer 1, where the underlying chain’s consensus is used.
Two documented rollup designs
An optimistic rollup treats submitted transactions as valid by default and provides a challenge period in which a result can be disputed using a fraud-proof process. A zero-knowledge rollup submits a summary of a proposed state change together with a cryptographic validity proof.
These are different validation approaches. They should not be presented as a simple safety or quality ranking, and details depend on the particular implementation.
What a rollup is not
A rollup is one kind of Ethereum Layer 2, not a name for every L2 system. It should also not be casually treated as interchangeable with a sidechain, validium or state channel. This article uses Ethereum’s terminology and does not provide instructions for using a rollup.
In short
Ethereum rollups execute transaction batches outside Layer 1 and post relevant information back to it. Optimistic and zero-knowledge rollups differ in how proposed state changes are checked.