What is an Identity ?

in identity you do not need to provide key value externally

auto-increment primary key field.

http://www.w3schools.com/sql/sql_autoincrement.asp

Identity (or AutoNumber) is a column that automatically generates numeric values. A start and increment value can be set, but most DBA leave these at 1. A GUID column also generates numbers; the value of this cannot be controlled. Identity/GUID columns do not need to be indexed. SELECT @@IDENTITY - returns the last IDENTITY value produced on a connection SELECT IDENT_CURRENT('tablename') - returns the last IDENTITY value produced in a table SELECT SCOPE_IDENTITY() - returns the last IDENTITY value produced on a connection