A cross-chain DApp is a decentralized application that is built to function across multiple blockchains, while a multi-chain DApp is a decentralized application that is built to function on a single blockchain. Cross-chain DApps are more complex, as they require interoperability between different blockchains, while multi-chain DApps are simpler, as they only need to interact with one blockchain. Cross-chain DApps are also more secure, as they are not dependent on a single blockchain, while multi-chain DApps are more vulnerable, as they are only as secure as the underlying blockchain.