0x is an open, permissionless protocol allowing for ERC20 tokens to be traded on the Ethereum blockchain. Developers can use 0x as a platform to build exchange applications on top of (0x.js is a Javascript library for interacting with the 0x protocol). For end users, 0x will be the infrastructure of a wide variety of user-facing applications i.e. 0x Portal, a decentralized application that facilitates trustless trading of Ethereum-based tokens between known counterparties.