Simple definitions for Primary Key, Unique Key and Surrogate Key --

Primary Key -
A primary key is used to uniquely identify each row in a table.
A primary key value cannot be NULL

Unique Key -
It is also defined similar to that of primary key with the following difference -
Unique keys are the keys where the implicit NOT NULL constraint is not automatically enforced, while for primary keys it ispresenet. Thus, the values in a unique key columns may or may not be NULL.

Surrogate Key -
A surrogate key is a artificial synthetic key which has same meaning through out the database. It is system generated key with no business value. Usually implemented with database generated sequences.

Hope this helps to some extent...!