SQL - Difference between Select Unique and Select Distinct

Select Unique col from
     (select col from table1 union select col from table2) alias

Select Distinct col from
     (select col from table1 union select col from table2) alias

Create Table Employee(
    Emp_PKey Int Identity(1, 1) Constraint PK_Employee_Emp_PKey Primary Key,
    Emp_SSN Numeric Not Null Unique,
    Emp_FName varchar(16),
    Emp_LName varchar(16)
)

Select Distinct Emp_LName
From Employee

select col from table1
union
select col from table2

select col from table1
union all
select col from table2