A unique key is a set of one or more than one fields/columns of a table that uniquely identify a record in a database table.
You can say that it is little like primary key but it can accept only one null value and it cannot have duplicate values.
The unique key and primary key both provide a guarantee for uniqueness for a column or a set of columns.
There is an automatically defined unique key constraint within a primary key constraint.
There may be many unique key constraints for one table, but only one PRIMARY KEY constraint for one table.