Difference between Primary Key and Unique Key

In SQL Server, we have two keys which distinctively or uniquely identify a record in the database. Both the keys seems identical, but actually both are different in features and behaviours.

  Define Primary key and Unique key

 CREATE TABLE Employee 

 (

 EmpID int PRIMARY KEY, --define primary key

 Name varchar (50) NOT NULL,

 MobileNo int UNIQUE, --define unique key

 Salary int NULL

 )  
Refer: diff b/n primary key and foreign key
  diff b/n primary key, unique key and foreign key