What is the difference between Primary key, Unique key, Sorrogate key?

Primary Key: A column in a table whose values uniquely identify the rows in the table. A primary key value cannot be NULL.
- There is only one Primary key for a table.
- Primary key can not contain null value.
 
Unique Key: Unique Keys are used to uniquely identify each row in an Oracle table. There can be one and only one row for each unique key value.
- There may be more then 1 unique keys for a table.
-  It can contain a null value.

Surrogate Key: A system generated key with no business value. Usually implemented with database generated sequences.