Basic difference is that all the data stored in the JCR are known as 'Node Store' while the resources stored in cloud or file systems are called as 'Data Store'. Refer [1] to know more details on the same
[1] http://docs.adobe.com/docs/en/aem/6-0/deploy/upgrade/data-store-config.html
You can achieve cluster mode only using mongoDB in AEM 6.0 and it can be configured in multiple ways. It can share same data or have their own farm which can be replicated. Refer [2] for multiple scenarios
[2] https://docs.adobe.com/docs/en/aem/6-0/deploy/recommended-deploys.html
