The 0x protocol utilized relayers to broadcast orders to the Ethereum blockchain. These relayers can broadcast orders through public order books, allowing anyone to submit or fulfill orders. Relayers can also do point-to-point orders which allow for maker (seller) to connect to a specific taker (buyer).
It is important to note that relayers are restricted to broadcasting these orders, but that they cannot execute the trades. So while relayers bring liquidity to the network akin to centralized exchanges, the value transaction of tokens takes place on-chain. Relayers are compensated with 0x’s native currency ZRX for processing the smart contracts and presenting them the Ethereum blockchain once a trade takes place.
Relayer diagram from the 0x whitepaper.
“Relayers host and maintain an off-chain order book in exchange for transaction fees. This diagram illustrates off-chain order relay and the sequence of steps used by Makers and Relayers to negotiate transaction fees in a trustless way. Transaction fees are moved from the Maker and/or Taker to the Relayer upon settlement of a trade”