Entity table belongs to Attribute Set. If column hydra_set_id is NULL, entity has all attributes which have been created for this entity (entity_type column's value should be the name of this entity, for example Product). Otherwise, entity has attributes which are assigned to this attribute set.
Attribute table keeps all information about attribute except its value.
entity_type - entity class name (Product, Category, etc)
name - attribute name
backend_type - database type of this attribute: string, text, integer, float, boolean, date time
default_value - default value for this attribute
white_list - determines whether a given attribute is allowed for mass-assignment
These tables are created for each entity. They store attribute values. Each table is designed to store concrete attribute types.