The 0x protocol, built on Ethereum, introduced a solution in the form of off-chain order books. While the execution of trades occurs on the Ethereum blockchain, giving users control of their funds until the exchange takes place, the order books are hosted by third-parties called Relayers.
These Relayers host and maintain order books and using the 0x architecture they can pool their liquidity together creating a more robust trading infrastructure. After submitting an order to the Relayer, a market maker waits for a taker to fill that order, at which point the trade is trustlessly executed on the blockchain.