Difference between Primary Key and Unique Key

PRIMARY KEY	UNIQUE KEY
It Accepts Only Unique Records. Duplicates won't be allowed.	Just Like Primary Key, Unique Key also accepts only Unique Records, It won't allow the duplicates.
It Doesn't allow Null Values.	It Allows Only One Null Value.
A Table Can have Only One Primary Key.	A Table can have more than One Unique Column.
By Default whenever we go with Primary Key it adds a Clustered Index.	By Default whenever we go with Unique Key it adds a Non-Clustered Index.