zkSync is a ZK rollup, a trustless protocol that uses cryptographic validity proofs to provide scalable and low-cost transactions on Ethereum. In zkSync, computation is performed off-chain and most data is stored off-chain as well. As all transactions are proven on the Ethereum mainchain, users enjoy the same security level as in Ethereum.
zkSync is made to look and feel like Ethereum, but with lower fees. Just like on Ethereum, smart contracts are written in Solidity/Vyper and can be called using the same clients as the other EVM-compatible chains.
zkSync stands out remarkably in security and usability among existing L2 scaling solutions. Thanks to the combination of cutting-edge cryptography and on-chain data availability, ZK rollups (the core network of zkSync) are the only L2 scaling solution that doesn't require any operational activity to keep the funds safe. For example, users can go offline and still be able to withdraw their assets safely when they come back, even if the ZK rollup validators are no longer around.