Difference between a primary key and a 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.

Types of Constraints

1.PRIMARY constraint
2.FOREIGN constraint
3.UNIQUE constraint
4.CHECK constraint
5.DEFAULT constraint