Primary key is a type of key where the data inside the
column should be unique and should not be null.we generally
use the primary key to access any of the rows of a table by
using the primary key column.when a table don't have a
primary key then we assign a primary key to the table which
is known as surrogate key for accessing any row of the
table. means the   key will act like a primary key for the
table.

Unique identitiy key is generated during data load process 
(ETL) in the datawarehouse system is called as Surrogate 
key.
It is mainly used to update slowing changing dimesion 
attribute data and also it removes dependencies on key 
originating in different data sources.

Primary key is uniquely identified each record in a table 
with no null values.