Primary Key vs. Unique Key

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.

SOURCE - http://www.learn.geekinterview.com/resources/interview-articles/sql-interview-questions-with-answers.html