A rollup is a blockchain scaling design that moves transaction execution away from a base chain while publishing batched data to it. Nodes can use that data to reconstruct the rollup's state. Combining compression with shared publication costs allows more activity without requiring the base chain to execute every individual operation.
How the layers cooperate
In the common Ethereum layer-2 model, users transact on the rollup, while Ethereum provides data availability and contracts for settlement. Optimistic rollups use a process for challenging incorrect state claims. Validity rollups, often called zk-rollups, submit cryptographic proofs that their proposed updates follow the encoded rules.
The location of the data matters as much as the proof mechanism. Keeping required data outside the settlement chain changes security assumptions; systems such as validiums should not be treated as equivalent to rollups using that chain for data availability.
What users experience
A rollup has its own network context, balances and applications. Moving assets between networks generally requires a bridge or another transfer service, even when the token names match.
A quick sequencer confirmation is distinct from base-chain settlement and withdrawal completion. Fees depend on execution, publication and the system's pricing rules. Congestion, unavailable operators, contract defects and upgrade permissions can affect service or assets. The rollup label alone does not establish decentralization or guarantee lower fees for every transaction.