In EAV(Entity-Attribute-Value) database model data is not stored in the single table. Data is stored in different small tables.
For example in magneto all the product information is not stored in a single table.
product name is stored in catalog_product_entity_varchar table
product id is stored in catalog_product_entity_int table
product price is stored in catalog_product_entity_decimal table
Magento use this technique for storing category,CMS content,customer etc information.
EAV database model has following advantages
I. Gives more flexibility in accessing,retrieving,storing the data and attribute.
II. EAV model enables easy upgrade .
III. It gives developer more control over the attribute and data stored in the database.