ZK-Rollups are a layer 2 scaling solution for blockchains. Hundreds of transactions made on ZK-Rollups are bundled in to one single transaction on the blockchain as a proof, after being checked by validators to ensure the proof is valid, a smart contract on the blockchain then verifies the transactions in the proof and once verified the transactions are submitted into the next block and are considered final. The smart contract execution data for these transactions will be stored on-chain. ZK-Rollups will be coming to Syscoin in Q1 2022 and will provide up to 210,000 transactions per second throughput.
Validium is a different form of ZK-Rollup, the difference lies in where the ZK-Rollup is storing its data. With a regular ZK-Rollup the data involved in the smart contract execution is stored on-chain, however Validiums store the data off-chain, for example using decentralized storage or a Data Availability Committee that has selected, trusted data safekeepers. This drastically increases the scalability. Validiums will be arriving on Syscoin in Q3 2022 and will bring up to 4,000,000 transaction per second throughput.