What is EAV in Magento?

Full form of EAV is Entity–attribute–value model.

EAV is a data model to data model to describe entities.
In EAV data are stored in different smaller tables rather than storing in a single table.

For e.g

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.