Counterparty Core is the reference implementation of the Counterparty Protocol, an extension to the Bitcoin protocol which implements a number of features that Bitcoin itself does not offer. These include token issuance, a fully decentralized and trustless asset exchange, contracts for difference, native oracles and trustless gaming. Counterparty works by ‘writing in the margins’ of Bitcoin transactions, and all Counterparty transactions are Bitcoin transactions with additional data that the Counterparty software can read and interpret.
See the official project documentation for more information, and for instructions on installing and running the Counterparty software.