Zcoin is the first full implementation of the Zerocoin Protocol, which allows consumers to have complete solitude via proofs. It's worth noting that Zcoin is irrelevant to additional cryptocurrencies using the Zerocash Protocol. Although Zerocash is a growth from Zerocoin, their various implementations are forks of one another, but rely on different cryptographic assumptions with various tradeoffs. Both methods supplement each other nicely, and a fantastic way to describe them would be sibling projects. The Zerocoin protocol is being researched and enhanced, like by a new extension that when implemented would eliminate the need for trusted setup and cut back proof sizes.