I am trying to learn magento framework. But it is hard to understand the database structure. So I am looking for a document that describe what each table exactly do and explain each column in that table. I have searched on google but all I found was magento database structure without any describes (table and column) Magento Database Diagram
Below link would solve all you queries related to Database Structure :
Below 2 links would show you the Database Diagram : http://inchoo.net/wp-content/uploads/2010/09/MAGENTO_v126.96.36.199-Database_Diagram.pdf
Its better if you go through whole Magento Knowledge Base Documents available first and then go for understanding the Database.
EAV tables are used to give us the flexibility of adding unlimited Attributes for Products, Categories, Customers etc at any point of project even without bothering to update the database.
You need to understand Stores, Eav Attribute Set, Product Attributes, Website as all of this values are stored in Tables to differ the values.