Both primary key and unique enforce uniqueness of the 
column on which they are defined. But by default primary 
key creates a clustered index on the column, where are 
unique creates a nonclustered index by default. Another 
major difference is that, primary key doesn't allow NULLs, 
but unique key allows one NULL only.

There is one more diff b/w primary key and unique key that 
there is only one primary key in a table but there are more 
then one unique keys in a table.

Primary can be used in foregin key but unique key does not 
used.

1)primary key uses clustered index
unique key uses non-clustered index

2)primary key doesn't allow null values
but unique allows one null value

3)there is only one primary key but more than one unique 
keys