Difference between Primary and Unique Key Constraint

Do you know what is the difference between Primary Key and Unique Key Constraint?

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I have got over 20 responses in my mail-box due to issues in posting the answers in the comment section and out of that selected "Aashish" as the winner of contest.

According to Aashish the Differences are:
1.Primary Key by definition cannot be null, where as unique key can accept null values but if the unique key is defined on a column which is not null , then this unique key can also be used as an alternate primary key functionality to identify unique rows in a table.
2.By definition you can have only one primary key defined on a table where as you can have multiple unique keys defined on a table
3.Also by default Primary key is created as clustured index and unique key is created as non clustered index.

This is the perfect answer to this question.