Uniquely identified each row in Table is called primary key. One table can have only one primary key (which can be combination of one or more than one columns), primary key can not allow null values and duplicate value. it is default cluster indexes.
Uniquely identified each row in Table is called uniquely key.Unique key does not allow duplicate values.Unique key allows only one null value in a column of table in which you have define unique key constraint.
Primary Key does not allow null values where as unique constraint allow ‘single’ null value.
A UNIQUE constraint is similar to PRIMARY key, but you can have more than one UNIQUE constraint per table.