what is the difference between unique constraint and primarykey and identifiers

[prasannas]

what is the difference between unique constraint and primarykey and identifiers?

[Paru]

Unique key allows null values atleast one time but primary key never allows null entries.

[debasisdas]

While a unique constraint enforces uniqueness (no duplicate value)
a primary key enforces not-null(value must be entered ) along with uniqueness.