unique key can be null but primariy key cant be null.

primariy key can be refrenced to other table as FK.

we can have multiple unique key in a table but PK is one and only one.

PK in itself is unique key.

Both primary key and unique key enforces 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.