I Have an doubt with unique, primary and foreign key?????

Question

Primary Key:-   

• The primary key is uniquely identifier in the table
• The primary key is not more than once in the table.
• The primary key needs a UNIQUE value and is not null.                                   

Foreign key:- 

• The foreign key is used to make link b/w more than one table.
• The foreign key is field/collection of fields which are primary key in another table fields.

Unique Key:-

• The Unique key is same like primary key provide feature of uniqness .
• All Value are different and unique in unique key fields. 

So my question is that----

• there are any simple definition to define primary, foreign, and unique key in one paragraph????
• What is different b/w Primary and foreign key????
• What is different b/w Primary and Unique key????
• What is different b/w Foreign and Unique Key????