Alfresco has its own standard database schema to store all its information. The database is the heart of the whole CMS system.
External applications, apart from Alfresco, should not perform any write operations in the Alfresco database directly. Always make sure you use the database for reading only. This is a standard practice. Don't make any changes to any values in the database. This schema is only required to understand Alfresco and troubleshoot.
As you know, everything in Alfresco is based around nodes, so the tables are also named in the same pattern. The database schema of Alfresco is very easy to understand. So, here we will not dive into all the tables, only a few important ones will be covered.