Zcoin is the first full implementation of the Zerocoin Protocol, which allows users to have complete confidentiality with Zero Information cryptographic evidence. It should be noted that Zcoin has nothing to do with other cryptocurrencies that use the Zerocash Protocol.
Although Zerocash is an improvement from Zerocoin, its respective applications are not based on each other’s simple forks, but rely on different encryption assumptions with different tradeoffs.
Both approaches support each other quite well and a good way to identify them would be sister projects. The Zerocoin protocol is actively researched and developed; for example, a new extension that, when applied, eliminates the need for reliable installation and significantly reduces proof dimensions.