Hi 

Basic Diffrence between primary key and unique key is the capability of unique key

for storing null values, a primary key on a column gurantees that the column must contain a value for a given row, but a unique key only guarantees that a column should be unique.

Apart from this difference , in oracle unique and primary key creates difference in

performance

e.g :  a primary key on a table enhance capability of using index

         on select count(*) from table_name  only if the table is analyzed

but a unique key won't