Unique key- unique key accept only one null value accept.but unique key use many time in table.
Unique: unique it allows null value only once.
Primary key: It is unique but it does't allow null value.
Below is a nice video which demonstrates the difference between unique and primary key practically.
in simple words...primary key is always unique and not null.but unique key is only unique it can be null.
From Microsoft SQL server doc: http://msdn.microsoft.com/en-us/library/ms191166.aspx