0x is an open-source protocol that facilitates the decentralized exchange of ERC-20 tokens on the Ethereum blockchain. The primary goal of the 0x protocol is to provide a standard set of rules and smart contracts that allow users to trade tokens directly with each other without the need for an intermediary.

Key features of the 0x protocol include:

1. **Decentralized Exchange (DEX):** 0x enables the creation of decentralized exchanges, also known as relayers, where users can trade ERC-20 tokens directly from their wallets without the need to deposit funds into a centralized exchange.

2. **Off-Chain Order Books:** The 0x protocol uses off-chain order books, allowing users to broadcast and discover orders without submitting them to the Ethereum blockchain. This helps to reduce congestion and minimize transaction costs.

3. **Liquidity Pools:** Market makers can contribute liquidity to 0x and earn fees for facilitating trades. This liquidity pool model aims to ensure that there are ample funds available for users to execute trades.

4. **Open Standard:** 0x is designed to be an open and extensible protocol. Developers can build on top of 0x to create their own decentralized exchanges or integrate the protocol into other applications.

5. **Interoperability:** Since 0x is built on the Ethereum blockchain, it benefits from Ethereum’s interoperability with other decentralized applications (DApps) and smart contracts. This allows for seamless integration with various projects in the broader Ethereum ecosystem.

6. **0x (ZRX) Token:** The 0x protocol has its native utility token called ZRX. ZRX holders have the ability to participate in the governance of the protocol by voting on proposed changes. Additionally, market makers may use ZRX to pay trading fees on certain relayers.

7. **Smart Contracts:** 0x uses smart contracts to execute and settle trades. These contracts include the Exchange contract, which facilitates the trading of assets, and the Token Transfer Proxy contract, which enables the transfer of tokens between addresses.

The 0x protocol plays a crucial role in the decentralized finance (DeFi) ecosystem by providing the infrastructure for trustless and permissionless token trading. It has been adopted by various decentralized exchanges and other blockchain projects seeking to enable peer-to-peer token exchanges on the Ethereum blockchain.