You can get enough information in the below Adobe Docs:
From AEM 6.0, We are using repository as OAK not TAR. Also, additionally you can use Mango-db as a additional repository
More information in this thread: Which option is best with AEM 6.2 between the TarMK and MongoDB
Also there is AEM GEMS Session on AEM with MangoDB: Running AEM on MongoDB
Difference between TAR and MANGODB: Adobe Experience Manager - CQ Tutorials: AEM Microkernels - MongoMK vs TarMK
Hope this helps!!