what is The Difference Between Unique key , Primary key

Primary Key and Unique Key difference :

In SQL 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 non-clustered index by default. Another major difference is that, primary key doesn't allow NULLs, but unique key allows one NULL only.

what si The Difference Between Clustered Index - Non Clustered Index

Refer Below Link :

http://www.sql-server-performance.com/2004/index-data-structures/