Data availability relates to the problem of how nodes in a network can be certain that a new block has been produced and that all the data in that block have been published to the entire network. This plays a major role in the consensus and liveness of the system. Data availability allows block producers to be held accountable for the data they post to the network and for creating new blocks and updating the consensus.
Both rollups and validiums are L2 protocols that post validity proofs to the mainchain to be verified by an on-chain verifier contract.
The difference is that rollups for data availability solutions have all data stored on-chain, and those for validiums have data stored off-chain. Which introduces additional security and trust assumptions to the system architecture outside the L1 mainchain protocol.
Orbis will be launching first as a validium with an off-chain data storage solution and then transitioning toward a rollup with minimal trust assumptions after data availability on Cardano has evolved.
The on-rollup block header data alongside the on-rollup UTxO balances and their corresponding authorized signing keys will be posted and stored on-chain in the rollup UTxO, which is the verifier contract. The current rollup state and on-rollup transaction history will be stored off-chain in a fully distributed database.