differences between primarykey and uniquekey

One of the basic question in any interviews is What is the difference between primary key and unique key?

Just give the below ponts, he/she will be very happy with the answers.

The Major differences between Primary Key and Unique Key are:

1) Primary Key doesn't allow null values, But Unique key does all nulls(Only One per key).

2) You can have No.Of Unique Keys on a single table, but you can have atmost only one primary key.

3) Primary Keys are used to define Referential Integrity Constraints.