Unique Key vs Primary Key

Unique key

Unique key constraint will provide you a constraint like the column values should retain uniqueness.
It will allow null value in the column.
It will create non-clustered index by default
Any number of unique constraints can be added to a table.

Primary key

Primary key will create column data uniqueness in the table.
Primary key will create clustered index by default
Only one Primay key can be created for a table
Multiple columns can be consolidated to form a single primary key
It wont allow null values.